FiberGlassics® would like to thank Jamil Mehdi for this article.  This content is copyrighted, and cannot be reproduced in any form without written permission from the author.


Boat Construction and Repair

“Back in the fifties, they didn't know how strong fiberglass was, so this boat,” says the man as he lovingly pats the fore deck of his boat, “this boat,” he continues, “was totally overbuilt.”

“Boats were built better back then. All of the fiberglass was hand laid,” says another man proudly.

“The hull is solid, it just needs a coat of paint,” explains another.


There is a pervasive myth in the boating world that early fiberglass boats were somehow better built than their modern counterparts. A myth that early boats are somehow impervious to the problems that have plagued the world of fiberglass construction in the decades following. A myth that we want so desperately to believe that we ignore reality.

I have never had someone bring me their boat and say, “This thing is built wafer thin.” or, “Every piece that can rot, has rotted.” or, “The hull flexes so bad underway that I feel like I'm riding on a Niagara Adjustable Bed.” or, “Every time I take the boat out, I'm half-surprised to make it back to shore.”

Yet this is the truth about most older boats. Still, because we didn't actually die, we convince ourselves that they're built like tanks when they're actually built like shower stalls.

But the nice thing about fiberglass is, no matter how far gone, no matter how rotted parts of the boat are, no matter how underbuilt it may be, it can always be repaired and it can always be improved. This section is about the common problems associated with older boats and how to affect those improvements.

Understanding Fiberglass Boat Construction

The first step in restoring a fiberglass boat is understanding how a fiberglass boat is built. In terms of classic fiberglass boats, this means understanding how they were built fifty or sixty years ago. To a large degree, not much has changed.

Once a boat has been designed, it moves from the pages of the naval architect to the production facility. The first step in a production line boat is the building of a plug. A plug, quite simply, is a sculpture of a boat. It is the shape from which all subsequent molds are built. It can be a solid form or it can be an actual prototype. It can be a wholly new design, or it can be a preexisting boat that has been modified.

Regardless of how the plug came into existence, it must be perfect. Since all future boats will be exact replicas of the plug, great care is taken to make sure the lines are fair, the finish is perfectly smooth, and the dimensions are accurate.

In today's world of fiberglass boat production, this is done with state of the art drafting programs and cut with million dollar milling machines to within 0.0001” tolerances. Fifty years ago, this was done by a guy named Rusty with a sanding block and a dust mask.

To look at the comparisons between the fine sloping, gently curving lines of a classic fiberglass boat, complete with tail fins, headlights, and any number of other unique qualities culled from the imagination of builder, and today's cookie cutter, almost indistinguishable, factory boats is to tacitly acknowledge the art of craftsmanship that is so glaringly absent in today's assembly line world.

Once the plug has been perfectly faired and polished, it is then waxed and treated with a mold-release agent. Gelcoat, which is simply heavily pigmented polyester resin, is then sprayed over the plug. When the gelcoat “kicks,” or begins to cure, wetted out fiberglass is laid on top in successive layers until it's reached the full desired thickness. The mold is then left to cure for sometime, usually about a week, before separating it from the plug.

Once the mold has been separated from the plug, the mold itself must be polished and any defects or tooling marks left over from the separation must be repaired. The mold is then waxed and treated with release agents and the boat building can finally begin.

The building of the boat is similar to the building of the mold, but it's built from the outside-in. First, pigmented gelcoat, this is the color you see when you look at a boat, is sprayed inside the mold. The gelcoat is air inhibited. This means, when it's exposed to air, it won't fully cure. This is done to assist the boat builder. The side touching the mold – the future outside of the boat – cures, while the side exposed to the air remains tacky. This is done so the builder can lay up the fiberglass directly into the partially cured gelcoat.

Layer after layer of wetted out fiberglass is added until the desired thickness is achieved. It is then popped out of the mold and the boat moves on to the next stage of the building process. Fifty years ago, this was done by hand. Sheets of fiberglass cloth, woven roving, and mat were cut to shape, wetted out with a brush or squeegee, and laid into the mold, one by one. This is known as hand-laid construction and it will be the default technique for the remainder of this book. Other methods of fiberglass construction, such as resin impregnation and vacuum bagging, will be discussed, but, for the avergage person, these methods are cost prohibitive.

As you can see, building a boat in this manner is like making a photocopy of a photocopy; fine details are sometimes lost, repairs are sometimes compounded, and the overall finished product can pay the price. And that is just for the first boat built from the mold. Over time, the mold will get used again and again until either the production ends or the mold is retired. If the mold is retired, a new mold must be made from the initial plug.

As you can imagine, the plug will begin to show signs of wear and tear with each successive mold. This must also be repaired prior to the new mold being laid up. Now you have a photocopy of a photocopy of a photocopy. This cycle continues on until the end of the production run.

For this reason, no two boats, even from the same mold, are exactly alike. I think this is part of what appeals to me. The challenge in classic boat restoration is in getting it as close to the original plug, the original vision of the designer, as possible. Perfection is unattainable. That line between perfect and imperfect is where art lives. It's where craftsmanship lives. Done well, it's something worth staring at for hours at a time.

Any boat more complicated than a canoe will require more than one mold. The hull and the deck will almost certainly be from two different molds. If there are more complex areas, additional molds may be required. Seats, splashwells, floorpans, stringers, and transoms may all have there own molds. In the end, they must all be tied together seamlessly for the finished boat to emerge.


Fiberglass – The History

The name says it all with fiberglass. It is, at the most basic level, glass fibers that are saturated in resin. Like rebar in concrete, when the resin cures, the glass fibers add tremendous strength to the mixture.

The concept of reinforcing a hardened structure with fibrous strands dates back 3,000 to Egypt where ancient clay shards reveal the use of glass fibers to increase the strength of the pottery. Although the strands were crude and coarse by today's standards, the process for making strands of glass by hand would be refined for the next few thousand years.

Fibrous glass was used by many different cultures across the world in the next few millenia, but because of the difficult and labor intensive method of production, its use was limited to decorative purposes. It's use as a reinforcement wouldn't be rediscovered until the nineteenth century.

The world of glass fiber production leapt with both feet forward into the industrial age in 1836, when a Frenchman named Dubus-Bonnel was issued a patent for using a loom to weave molten glass.

34 years later, a man named John Player refined the process by using a steam jet to mass produce glass fibers. These fibers, known as “mineral wool” were found to have excellent fire retardant and insulating properties and is still used today for, among many other uses, car muffler and electrical insulation.

10 years after that, Herman Hammesfahr was awarded a patent for a type of hybrid fiberglass and silk cloth that was found to be both heat resistant and strong.

The method for producing fiberglass that we still use today happened, like most technological advances, by accident in 1932. A researcher for Corning Glass named Dale Kleist was trying to fuse together two glass blocks when an errant stream of compressed air hit the molten glass. The result was the same hair-thin strands found in modern fiberglass.

Over the next few years, fiberglass would evolve into the product we know today.

In 1935, Corning Glass, in a joint venture with the Owens Illinois company, introduced a product known as “Fiberglas”, and in 1936, Carlton Ellis of Dupont was awarded a patent for the first polyester resin.

In 1938, Corning Glass and Owens-Illinois would merge to become Owens-Corning and lead the world in the production of fiberglass and fiberglass products. Initially, Owens Corning used fiberglass to make airplane parts for the war effort, but it was one of their own, a man named Ray Greene, who began experimenting with composite boat construction. His first composite boat was in 1937 and he built the first modern fiberglass boat in 1942.

After the war and for the next ten years, fiberglass came into wide acceptance, quickly usurping wood as the preferred material for boat construction.

In older boats, the fiberglass and resin cocktail was limited to three different weaves of fiberglass and one kind of resin. Boat builders would alternate between weaves for each layer and wet it out with a brush dipped in a jug of resin. It was a simple formula with no real options for deviation. They would keep adding layers of glass and more resin until the piece has reached the desired thickness. (Though it should be noted that Pearson Boats experimented, unsuccessfully, as early as 1951 with a form of vacuum bagging).

Today our options are an embarrassment of riches and, at times, overwhelming. While the basic concept of fiberglass hasn't changed for the last sixty years, the choices of resins, glass, composites, and ancillary supplies have exploded in recent years. Before getting into the repair part of this section, it's important to understand what these options are and how they're used.


Fiberglass and Composite Materials


As mentioned above, classic fiberglass boats used only three different weaves in their construction. Today there are more options available. Whether you choose to stick to traditional construction methods to keep your boat as authentic as possible or whether you choose to take advantage of the many improvements to the science and understanding of composite fabrics is entirely up to you.

Fiberglass Cloth

Fiberglass cloth has the highest strength to weight ratio of all glass fiber fabrics. I want to specify the “glass” part of that last sentence because there are other composite materials, such as graphite, carbon, and kevlar that are actually much stronger. As far a glass goes, cloth is the strongest as compared to weight. I also want to stress the “strength to weight” part of that sentence. Cloth is the strongest compared to weight, but that doesn't mean that it's the strongest layer of fiberglass you can lay into a boat. No, that title goes to woven roving which is also considerbly heavier.

Yet, for some reason, probably because of the diminutive nature of the fabric, it is almost entirely used improperly. Fiberglass cloth excels as the stuctural core of a thin, yet strong laminated panel. Alternating between cloth and mat, this fabric will end up as a strong and light weight boat.

Cloth, being a much lighter and drapier fabric, conforms to odd shapes much better than roving. For this reason, cloth is routinely, and wrongly, used as a “skin” over wood and plywood. One of the most common mistakes made, by both the boat builders of years past and the Do-It-Yourselfer of today, is wetting out fiberglass cloth, slapping onto a piece of wood, and believing this will prevent moisture from penetrating the surface. This is about as effective as an umbrella made out of gauze.

Fiberglass cloth is nearly pointless without being sandwiched between two layers of mat. The cloth is made up of thin strands of glass in alternating directions. The result is a strong fabric punctuated by a million little holes for water to penetrate.

Because it conforms easily to curves and because it's the easiest of the three popular weaves to use, fiberglass cloth is commonly used in 4” and 6” strips, called tape, to tab interior structures such as floors, seats, and cabinetry to a boat.

Mat, or Chopped Strand Mat (or CSM)

CSM, also called “Mat”, is what most people think of when they hear the word fiberglass. Mat is a mish mash of glass strands glued together to make sheet. This “weave” of fiberglass is the best for what it's supposed to do and the worst for what it isn't supposed to do.

Sadly, I see this used improperly most of the time, usually resulting in costly repairs. It's essential that you understand its intended purpose and limitations.

Mat, (as I will continue to refer to it for the remainder of this book), is not intended to add strength to a fiberglass boat. Many times I've seen the DIYer try to improve a structural aspect of his boat by adding a layer or two of mat. This is only slightly better than doing nothing at all. I will explain this statement in further detail in the subsequent descriptions of fiberglass fabrics, but for now, just know that mat is not structural.

The actual purposes of mat are manifold.

First, because of it's overlapping multi-directional strands, it is the most waterproof of all the fiberglass fabrics. For that reason, it is considered a “finishing” fabric. In other words, it's the layer of fiberglass that is closest to the exterior of the fiberglass lay up. If water should penetrate the gelcoat, you want the first layer of fiberglass it meets to be mat.

Second, mat conforms to shapes more easily than all other fabrics. This is a direct result of the glass fibers being only several inches long and multi-directional. These fibers are held in place by a binder that dissolves when it's wetted out with resin. Once the binder dissolves, the glass fibers will conform to whatever shape they are laying next to. Because cloth and woven roving are woven in alternating directions, they want to lie flat. When you try to convince them to turn a corner or conform to an irregular shape, they have a tendency to pull away from inside corners and lift up from outside corners. When cloth or roving is sandwiched between two layers of mat, the mat is going to help convince them to stay where you want.

Third, mat acts as a sort of gasket between layers of cloth or roving. When laying up layers of fiberglass, you never want to put a layer of cloth or roving directly on top of another layer of cloth or roving without a layer of mat between them. Imagine the amount of surface area that would be in contact by pushing together two irregularly surfaced items such as two pieces of diamond plate. They will not touch except at the high points of each surface. Now throw a blanket in between them and you get almost complete surface contact. Mat is the blanket.

Woven Roving

Woven roving is similar to cloth in that it is a woven fabric. Where fiberglass cloth looks like the kind of material you could feasibly make clothing out of (very itchy clothing), roving looks more like wicker. It is far bulkier than cloth and, as a result of it's thickness, it is the single strongest layer you can lay up.

Roving is most often used in the super structure of a boat where thickness is most important, such as the hull, deck, and transom. Where cloth is the Popeye of boat building, roving is Bluto.

Roving is also the hardest of the fabrics to get to conform neatly to complex shapes. Because it's made out of thick bundles of glass strands, it is best used for large flat or slightly curved areas.

Bi-axial Glass

Woven fabrics have a downside. Like wicker, the weft and weave of the fabric must first go over one perpendicular strand, then under the next, then over, then under. The result are glass fibers that do a lot of bending in order to make a fabric. At each bend; each hill and valley of the weave, the glass is stressed. At a microscopic scale, this causes some of the individual fibers to crack. When this is compounded over the entirety of the surface, the result is an overall weakening of the fabric.

Bi-axial glass is not woven. It is two separate layers of glass laid on top of each other that run in opposite directions. The top layer is made of parallel strands of glass running 45 degrees to one side and the bottom layer is made up of parallel glass strands running 45 degrees to the other side. Instead of being interwoven, they are laid on top of each other and stitched together with a binding thread.

On first glance, this sounds like it's a vast improvement over woven fabrics. In fact, it is stronger to some degree, but it is also much more expensive than cloth or roving.

But is it worth it?

Side note: This is where people are going to come out of the woodwork to tell me I'm wrong. These next few paragraphs are considered blasphemy in the marine construction trades and I'm fully aware of it. I also want to stress, that the next few paragraphs are based on my interpretation of the science. It is my opinion, not accepted fact. Do with that what you will.

Bi-axial glass has good points and I do use it for a number of reasons. Strength is not one of them. The benefits of bi-axial cloth are, I believe, grossly overstated and, in some respects, simply non-existent or, in certain regards, detrimental.

The marketing behind bi-axial glass always trumpets two main factors, strength and finish. Strength because the glass is not compromised by having to bend up and down as woven fabrics do, and finish because, since the top layer lays flat, there is less of a three dimensional profile that needs to filled and faired in order to get a perfectly smooth surface to apply paint.

Here are my problems with bi-axial glass:

First, look closely at a swatch of bi-axial glass. Is it an even layer of glass strands laying perfectly flat on top of another layer of going in the opposite direction? No, it isn't. In fact, it's small bundles of glass laid next to each other all going in the same direction. That's not the same thing. These small bundles of glass look about the same as a strand of woven roving.

As I mentioned earlier, you can't laminate woven roving on top of woven roving without a layer of mat between them. The reason being you don't get total surface contact between the layers. Bi-axial glass is comprised of two layers of the same thick glass strands running in opposite directions, but there is no binder between them. Because the alternating strands of woven fabrics are mechanically interlocked by the weave, they don't require a layer of mat between them. When bi-axial is wetted out, the two layers compress at the high points of each strand, but without a mechanical bond or a layer of mat between them, the structural improvements of straight glass strands are negated by an increased chance of delamination.

Second, the improved structural qualities of straight glass strands over glass that must bend in order to be woven are almost completely canceled out having to stitch a binding thread through the bi-axial fabric. This thread, which is stitched every 1/8” to keep the fabric from falling apart, compresses and deforms the glass fibers as much, if not more, than the weaving process.

Lastly, the benefits of a smoother finish are simply not true. As you'll read later in this section, no woven or multi-directional fabric should ever be used as a finished layer. Mat should always be the first and last layer of a fiberglass lay up schedule because it is the least porous (most waterproof). But even if that wasn't the case, the smoother finish of the unidirectional top layer of bi-axial glass is completely negated by the high profile of the binding thread. Even if it dissolves in resin, the thread imparts a crimp to the strands which never disappears completely.

On the whole, bi-axial glass is more about marketing than it is about real improvements to the marine composite world. It is two to three times more expensive to buy, yet costs the manufacturer roughly the same price to make. It is a much higher profit margin with only limited benefits to the end user.

So what are the actual benefits? I mentioned earlier that I do use bi-axial glass in some situations. Before I tell you how, I want to explain why. There are a couple of reasons, one of them is good and one of them is bad.

The good reason: Bi-axial glass has much better shear strength than other weaves. What does this mean? Boats twist. The front of the boat is pushed one direction while the back of the boat is pulled the other direction. This is called flex and it's a totally different thing than shearing. Flex is what leads to delamination. A hull flexes. A deck flexes. A floor flexes. These are all areas where the increased chance of delamination I associate with bi-axial glass would be a detriment.

Shearing is the back and forth movement of two perpendicular pieces. Shearing occurs where the floor meets the hull. Shearing occurs at intersections of opposing panels. Because of the opposing 45 degree angles of bi-axial glass, it resists shearing much better than the up and down, left and right strength of woven fabrics. I use bi-axial glass in lieu of cloth when tabbing in pieces to a boat, whether it's a floor, a bulkhead, or interior seating. this is the one arena where shear strength supersedes flexural strength.

The bad reason: As a professional, I have a vested interest in understanding the working properties of the materials I use. My customers do not. I will try to explain my reasoning, but if a customer doesn't understand the mechanics of fiberglass, all he's going to hear is that I'm using a cheaper fabric. I can spend hours trying to explain myself, but if he doesn't get it, he's going to walk away thinking I'm trying to cut corners. If, after explaining myself, a customer insists, and I don't believe it's use will compromise the overall integrity of the boat, I will let him throw his money away.


Fiberglass Weights

When you purchase fiberglass fabric of any kind, you will need to know how to ask for what you want. Because nothing in the boating world is allowed to be either straight forward or simple, the rulers of the fiberglass kingdom have decreed that cloth and roving should be identified by their respective weights in square yards, and that fiberglass mat should be identified by its weight in square feet. This is the dumbest rule ever and leads to near total confusion for the uninitiated.

This is like going to a lumber yard and having the guy behind the counter tell you that teak is sold in board feet, but mahogany is only sold in meters. So when you go to your fiberglass supplier for the first time don't accuse the guy of trying to pull a fast one on you when your 1.5 oz mat actually weighs more than four times as much as your 3.2 oz cloth and over half as much as your 24 oz woven roving.

By the way, those weights are just about the industry standards for boat work. You will often have a choice of heavier or lighter fabrics, but for general use those are the numbers you would use. If you do require heavier or lighter fabric, just remember that they must all be heavier or lighter equally. In other words, wafer-thin mat (0.75 oz) is not going to act as a very good go-between for extra heavy layers of woven roving.

Resin to Fiberglass Ratios

You have to remember how fiberglass fabric is weighed in order to get a good resin to glass ratio. The rule of thumb for wetting out fiberglass cloth and roving is 50/50. This means that if you are laying up one square yard of 3.2 oz fiberglass cloth, you'll need 3.2 ox of resin to wet it out properly (that's 3.2 oz by weight, not 3.2 fluid ounces). If you have to wet out half a square yard of 24 oz roving, you'll need 12 oz of resin.

Mat is a little bit different. You have to use twice as much resin with mat. That means, if you have to wet out a square yard of 1.5 oz mat, you would have a total of 13.5 oz of mat. For a 2 to 1 ratio, you would then need 27 oz of resin to do the trick



Polyester Resin

Polyester resin is a viscous adhesive that, when catalyzed with Methyl Ethyl Ketone Peroxide (MEKP), creates a hard and brittle amber-like solid. When mixed with strands of glass, this becomes a durable construction material that, by weight, is stronger than steel.

Early fiberglass boats were constructed almost exclusively of polyester resin and glass, and most production line boats are still made that way today. Polyester is the most widely used and available resin on the market today. It is also the cheapest, and, not surprisingly, it is also the weakest, the most porous, and shrinks more than any other resin.

I don't mean to give the impression that polyester resin is so far below the alternatives that it shouldn't be used. If that were the case then this book wouldn't exist. I think it's a testament to the qualities of polyester resin that, fifty years later, the fiberglass portions of the boats we choose to restore are still the strongest element left.

But what is it?

The following excerpt was taken from Wikipedia. If you understand it, please call me and explain it to me using simple words.

Polyester resins are unsaturated resins formed by the reaction of dibasic organic acids and polyhydric alcohols...

“Unsaturated polyesters are condensation polymers formed by the reaction of polyols (also known as polyhydric alcohols), organic compounds with multiple alcohol or hydroxy functional groups, with saturated or unsaturated dibasic acids. Typical polyols used are glycols such as ethylene glycol; acids used are phthalic acid and maleic acid. Water, a by-product of esterification reactions, is continuously removed, driving the reaction to completion. The use of unsaturated polyesters and additives such as styrene lowers the viscosity of the resin. The initially liquid resin is converted to a solid by cross linking chains. This is done by creating free radicals at unsaturated bonds, which propagate in a chain reaction to other unsaturated bonds in adjacent molecules, linking them in the process. The initial free radicals are induced by adding a compound that easily decomposes into free radicals. This compound is usually and incorrectly known as the catalyst. Substances used are generally organic peroxides such as benzoyl peroxide.”

I'm not going to lie to you. I don't know what that means. I've tried to understand it, but the more I try, the more I want to slam my head into a wall until I'm unconscious. This is one of those things where I don't know what I don't know, and I don't care.

This is what I do know. Not all polyester resins are equal. The stuff you buy at the auto parts store is not the same stuff that's intended for use on boats, (you will find this to be a recurring theme in this book). There are two different kinds of polyester resin: Orthophthalic and Isophthalic.

If you want to have some fun, go into an auto parts store and ask the guy behind the counter if his polyester resin is orthophthalic or isophthalic. The guy will first look confused, then he will ask his coworker if their resin is “Ortho-phallic”. When his coworker shrugs and goes back to the customer he was dealing with, a customer who, Thank God, only had a question about spark plugs, your guy will look back at you and say something profound like, “It's for fiberglass.” My point is, the days of relying on the expertise of the guy behind the counter are gone forever. It's up to you to know the difference.

Isophthalic resin was used on most boats prior to the 70s. These were known as the “Pre-Blister Years”. In the 70s and 80s, (really the finest time in American History for giving us all forms of polyester, pet rocks, and disco), the move was made to orthophthalic resin. Not because it was better, but because it was cheaper and easier to use. Ten years later, repairing the blisters caused by the inferior resin became a cottage industry.

It turned out that isophthalic resin was much more solvent resistant. Water, it seems, is an excellent solvent. The water would degrade the resin and penetrate, first the gelcoat, then the layers of glass behind it, resulting in osmotic blisters, colloquially known as boat pox.

Once it was discovered that inferior resin was the culprit, most of the industry shifted back to isophthalic resin. Some went in other directions...

Today, polyester resin is commonplace. It can be found at local hardware stores, big box stores, marine chandleries, and auto parts stores. It is the basis for most marine and automotive fillers and fairing compounds, and it's easily used.

The advantages of polyesters over other types of resins are that it's easy to find and, because it is cheaper than it's counterparts, it lends itself to the home user as a product that won't break the bank if you screw it up. It also has an advantage over epoxy in that it's cure time can be sped up or slowed down by adjusting the amount of catalyst used.

Epoxy Resin

Epoxy is widely viewed as the more talented younger brother to polyester resin. It is stronger and more water resistant. It is chemically compatible for repairs to polyester based construction and, when mixed with any of a wide variety of fillers, it makes an excellent adhesive, filler, or fairing compound. It is also compatible with many other substrates used in the marine industry such as steel, aluminum, and wood.

At about the same time as the polyester resin revolution was just gathering steam, epoxies were also quietly building momentum.

In 1909, a Russian organic chemist named Nikolia Prileschajev (Prilezhaev) discovered a method for epoxidation. Epoxidation is, simply put, the joining of an oxygen atom to two other atoms that are already joined. The (now obvious) implication of this process was that a viscous resin could could be molecularly altered to turn into a hardened solid. While epoxidation occurs in nature, the ability to replicate the Prileschajev Reaction, as it came to be known, in a lab would not be done successfully for decades.

In 1936, Dr. Pierre Castan of Switzerland and Dr. S.O. Greenlee of the United States, almost simultaneously, (and who share equal credit for the discovery), were able to successfully synthesize epoxidation. Dr. Castan's research was done for the Swiss company, Ciba, Ltd., who, in 1946 with the release of the world's first commercially available epoxy, would go on to become one of the global leaders in epoxy production.

In the years that followed, epoxy gained notoriety as an excellent adhesive that was capable of bonding to wood, steel, aluminum, fiberglass, and many other materials. It had gained a niche market in the boating world as a superior adhesive for wooden boat construction. But it hadn't gained wide acceptance as a laminating resin because of expense and it's limitations as a coating.

By all accounts, the Gougeon brothers, Jan and Meade, revolutionized the world of epoxy by starting West Systems. Although the company wasn't founded until the 1970s, The Gougeon brothers had been working and experimenting with epoxies since the late 1950s. It was their work that brought a workable, easily used epoxy to the mainstream of boat construction and repair.

Today, the epoxy industry has evolved into a competitive world that is increasingly user friendly, price-conscious, and adaptable. Epoxies are now customized for specific purposes, from laminating fiberglass to below-the-waterline barrier coatings to finished surfaces.

While epoxy is a windfall for the marine industry, it is not without its drawbacks. The two most glaringly obvious are: Its price, it is still the most expensive of the choices available for resins, and its limitations for use with other resins. Specifically, epoxy will adhere like a dug-in tick to all other cured resins, but other resins don't adhere well to cured epoxy.

Other drawbacks include shrinkage – while polyester shrinks the most in the curing process, epoxy is not far behind – and cure times. Polyester resin cure times can be accelerated or retarded depending on the amount of catalyst added, but epoxies require exact mixing ratios of resin to hardener. To alter the cure time of epoxy, it's necessary to buy a faster or slower hardener.

Finally, epoxies, like a professional athlete with a nagging hamstring injury, are handicapped by amine blush; an annoying byproduct of all epoxies, but it's most noticeable in cheaper epoxies and with some slower hardeners. Amine blush is a waxy hazy film that develops when curing epoxy is exposed to moisture or humidity.

In extreme cases, it can cause the the surface to become a milky white and chalky, but even if you don't see it, it's there. This means that cured epoxy must be cleaned either with soap and water or, in extreme cases, by sanding and solvent wiping, before the epoxy can be coated, either with a finish or with more fiberglass.

Vinylester Resin

I'm going to start this with a baseball analogy for two reasons; first, I like baseball and it pleases me, second, I've found that baseball works as an analogy for everything.

Polyester is the utility infielder of the boat building world; good at most things, but not great at any, but still able to be had for a decent price. Epoxy, on the other hand, is the Ichiro or Ken Griffey Jr. (yes, I'm a Seattle Mariners fan. New York and Boston can bite me). In other words, epoxy is the star; expensive and worth it. It puts up the big numbers, it leads the league in hits and home runs.

But where does that leave vinylester?

Vinylester is the finesse player. Vinylester is the Greg Maddux of resins. Unimposing to look at, yet, day after day, year after year, it quietly wins every game until, at the end of it's career, you look back and realize it's going into the Hall of Fame. Undervalued, under reported, and underestimated, vinylester is the inexpensive best of both worlds.

The name implies that it is closely related to polyester resin. For the purposes of the amateur boat restorer, it is. It is catalyzed the same as polyester, it's chemically compatible with polyester, and the cure time can be adjusted like polyester. It has all of the good points of polyester, but few of the drawbacks. It is more waterproof, It shrinks less than both polyester and epoxy, and it is relatively inexpensive.

Technically, it is a hybrid of polyester and epoxy. I'm hesitant to say that because, even though it's true, it might give the reader the mistaken impression that it is equally compatible with both. It's not. Like polyester, epoxy will adhere very well to it, but it doesn't adhere very well to epoxy. As far as I'm concerned, this drawback is grossly outweighed by its advantages.

Vinylester resin finds its roots in the aftermath of the osmotic blistering age. The boat building world was looking for both the cause and the solution to the problem. Vinylester resin was experimented with as a barrier coat between the porous gelcoat and first layers of fiberglass behind it, and it did so with great success. Because it is still, and always was, moderately more expensive than polyester resins, it's use as the primary resin for fiberglass lamination didn't catch on with the mass-production builders.

These days, vinylester built boats are considered to be a mark of high quality, and rightfully so. With a price that is far below most epoxies, I believe this is the best resin available for at-home boat restoration.

Ultimately, the resin you choose to use is going to be based on your comfort level, your pocketbook, availability, and whimsy.


Fiberglass Fabrication

I wasn't sure whether or not to add this section to the book because fiberglass fabrication (as opposed to repair) is generally not associated with boat restoration, but after serious thought, I decided its omission would close the doors to an entire subset of creativity.

Before I start talking about how to build something new, I want to clarify what I meant by that last paragraph. It's certainly important to know how to fabricate new pieces for an incomplete boat. It's also important to know how replace pieces that are so badly damaged that repair is not an option. Those two situations alone are reason enough to add this section. But there is another reason. A reason that goes largely untalked about. A reason that is looked upon with disdain by many and as sacrilege by some. That reason is customization.

There are some boats that are considered untouchable. The Glass Slipper, the Lonestar Meteor, the Bell Boy Banshee, the Herter's Flying Fish, and many others in the eyes of their followers; these are boats that would inspire anger bordering on violence if we were to come across one that had been defiled by an amateur who decided to completely alter the original intent of the designer.

Then there are the others. I'm not going to name names because to mention a specific boat would inevitably piss someone off, but we've all seen them. A fifty year old derelict sitting in someone's back yard, for sale for a hundred bucks. These boats don't have the cult-like following. They may not have the tail fins or the unique hardware or the headlights or the... These boats are cheap and neglected because they weren't blessed with a creative designer. But does that mean these boats should be relegated to the scrap heap? Does that mean these boats aren't worthy of restoration? Absolutely not.

The customization of classic cars using whatever materials are laying around the shop have come to be known as Rat Rods. Sometimes the customization is necessary because the vehicle is too far gone to repair. Sometimes it's because the artist, (and I do consider them art), doesn't have the money to restore them properly. And sometimes it's because it's just fun to see what you can make from nothing. Regardless of the reason, whether by necessity or by creativity, without them, the world is a duller place. Without them, we would never have Jesse James, American Chopper, or Overhauled. Without the artistic eye of a man on a budget, our roads would be limited to a sea of indistinguishable sedans and SUVs. Love them or hate them, we always notice them.

But for some reason, that creativity has gone largely ignored by its marine counterparts. I, for one, would love to see more customized, one of a kind entries into the classic boat world. And with the qualities of fiberglass, there is no limit to the creativity afforded by a guy with $100 and too much spare time.

In fact, I find it ironic that the things we love most about Glassic boats; their originality, their wild divergence from the accepted norms of nautical history, is so closely bound by an unwillingness to take design risks in their restoration. If we, as a group, don't move beyond the hoarding mentality, then we've betrayed their spirits, they might as well be animals in a zoo.

To be clear, I have a Bell Boy Banshee. I love it. I think it's perfect. I wouldn't change a thing about it. But everyday, I see disregarded featureless boat offered virtually for free. I never pass by one without thinking, I could make that boat sing. I could make that thing the wildest boat anyone has ever seen. I could make that boat look like a cartoon, and the only detriment to the world would be one less boat in a landfill.

So, while this section is necessary for some who need to learn how to restore their boat to its original condition, it is my sincere hope that some who read this will be inspired to take a risk and create their own masterpiece.

Making Plugs and Molds

It is not always necessary to make a plug or a mold when fabricating new construction. Typically, they are used for production runs of more than one piece. However, building a plug or mold is both an educational experience and a gratifying accomplishment.

While this was quickly covered in the introduction, I'll go into more depth here and try to help the reader from making some of the more common mistakes.

It may be pedestrian, but it's important to note the difference between a plug and a mold. Both are simply a negative shape, the inverse of the piece you want to make. A plug is used to lay up fiberglass on the outside of the shape and a mold is used to lay up fiberglass on the inside of the shape. It can be structurally solid, as when used for long production runs, or it can be built for a single use. It is up to the reader to decide for himself how sturdy or how flimsy a mold needs to be.

First, a plug or mold doesn't need to be built like they do in the factories. It can be cardboard, it can be Tupperware, it can be a bowling ball, it can be plywood, a tee-shirt, your wife's fine china, your kid's iPod, a solid block of wood, or made from an existing piece that you want to duplicate. It can be made out of whatever is available that will create the shape you choose.

For the purposes of this section, I'm going to pretend that we're making both a plug and a mold for a battery box. It's a simple shape and easily described. But now matter how complex your mold or plug may be, the process doesn't change. Whether you're making flat panel or a fiberglass representation of the Chrysler Building, the steps remain the same.

For our battery box, obviously we need to know how big to make it. This isn't quite as simple as measuring the battery and building a box that will accommodate it. We have to take into account the thickness of the final product. If our finished battery box is going to be made out of a quarter inch thick fiberglass then we have to alter the dimensions of our plug or mold to take that into account. If we are making a plug, then we have to build it to the inside dimensions of the finished box because the box will be built from the inside out. If we're making a mold, we have to build it to outside dimensions because it will be built from the outside in. Since deep-cycle marine batteries are very difficult to make smaller, you probably want to get this right the first time.

Once you have your dimensions, it's time to start building the box shape. This can be made out of any material you want, from plastic to plywood, but it's important to know, if your goal is a smooth finish, using a smooth material will make the process much easier. For this reason, a finished surface material such as hardiboard or masonite will speed things up. If you use plywood, try to use a finish grade or sand the surface smooth prior to construction of the box. Since plywood is common and cheap, I'm going to continue this section under the assumption that it is the medium of choice.

The box doesn't need to be built like furniture. It doesn't need to be of dovetail construction or have interlocking miters. Remember, you building a shape. It should, however, be strong enough to withstand working on it without deforming or collapsing. Gluing and screwing it together will be more than strong enough.

If you're making a mold, it doesn't matter how thick the plywood is so long as it's rigid. If you're making a plug, you will need plywood that's at least half an inch thick. I'll explain why in just a second.

Once the box is built, you need to fillet the corners of the mold. Filleting means rounding over the inside corners. As mentioned earlier, some wetted out fiberglass fabrics don't conform to tight corners well. Mat will conform to complex shapes most easily, woven roving will conform with the most difficulty, and fiberglass cloth is somewhere in between. When you're laminating multiple layers of fiberglass, you'll be using mat and at least one of the other two fabrics.

If you try to build up layers of fiberglass against a sharp 90 degree angle, the mat will do OK, but the subsequent layers of cloth or roving will either pull away from inside corners or lift away from outside corners. The result is delamination at every corner. That's why it's important fillet all of the inside corners and round over all of the outside corners.

There are a number of products that can be used for filleting. You can use Bondo or another fairing compound. You can use a polyurethane sealant such as 3M 5200 or you can sculpt it out of wood. My recommendation is Bondo. It cures quickly and it can be sanded to give a nice finished shape. In the end, it doesn't matter what you use so long as you get the shape right and it stays put.

If you're making a plug, you'll need to round over each outside corner. This is why you need plywood that's at least half an inch thick. If the plywood is too thin, you won't be able to round over the corners enough.

Whether filleting or rounding over, the larger the radius, the better in terms of fiberglass lamination. If you're not making a battery box, but instead making a more complex shape, you will probably have both inside and outside corners to deal with.

With the mold or plug now built, It's time to fair the mating side. Your finished product will only be as smooth as your form. If you want a glass smooth finish as soon as it's popped out of the mold, then your mold must be glass smooth. This is where experience is going to come into play.

When boats are built, great care is taken to ensure the mold or plug is absolutely perfect. This is because dozens, if not hundreds and sometimes thousands of boats will be made from it. If you are building a mold or a plug in your garage for a single use, you have to ask yourself which is easier, perfecting the mold (or plug), or perfecting the product once it's out of the mold. The answer is not the same for every situation.

Here are my personal thoughts. In order to get a beautiful glossy gelcoat finish on any one-off molded piece of fiberglass, a great deal of sanding, compounding, and polishing is going to occur at some point in the process. Whether you spray (or brush) gelcoat on the inside of a mold, the outside of a plug, or directly onto the fiberglass that been popped out of the mold, there is no escaping this procedure. The only variable is what is easiest.

In terms of our battery box, we have to imagine our finished product. Where is the battery box going to live? What part of it will be visible? This is what defines if you make a mold or a plug and which side is the glossy side. If the battery box is going to live on the floor in the boat back by the transom for all the world to see, then naturally you'd want the outside of the box to be the finished side. But maybe you want to cut out a square in the floor and drop your new battery box inside of that hole, in that case, the inside of the box would be the part you see.

So how does this information help us make our decision? Well, it's a helluva lot easier to sand the outside of something than it is to sand the inside of something. Take a cardboard box and a sanding block and see how much mobility you have when you mimic the sanding motion on the inside of the bottom of the box. Now flip the box over and mimic the sanding motion on the outside of the bottom of the box. I think you'll agree that sanding the outside is a lot more effective.

If our box is going to be shiny on the outside, it will be easiest to build a mold and add gelcoat to the finished product after it's been removed. If we want the inside to be shiny, it will be easiest to build a plug, spray the plug with gelcoat and polish it up, then, when the polished plug has been duly waxed, spray on more gelcoat and laminate our fiberglass on top of that. When we separate the two, we will have a mirror image of the polished plug. As you can see, neither situation requires sanding and polishing the inside of the box beyond the filleting stage.

Side note: There are two factors which are vital to the success of your project.

First, if you intend to add the gelcoat after the mold, it is vital that you seal the mold prior to fiberglass lamination. If your mold has any porosity whatsoever, the resin used in the layup will fill those pores and make separation of the mold impossible. Sealing the mold with several coats (or more) of spray lacquer is essential

Second, a mold or plug must have a taper in order for you to remove it from the work piece. If it does not grow wider toward the open end, you will have to make the mold (or plug) in two pieces and glass them together.

The exception to this thought process is when you are making multiple pieces from the same mold or plug. In that case, it's easiest to go through the hassle of polishing the mold once, regardless of how difficult it may be, rather than doing each piece individually.

At this point, the mold is built. All we have to do now is prepare it for the fiberglass lay up.

The next two steps are waxing the mold and treating it with a mold release agent. Waxing the mold requires a mold release wax. Partall #2 is available through most fiberglass suppliers. It is a special wax that is designed to be compatible with fiberglass resins and the release agent (PVA). The mold must be waxed multiple times with a minimum of one hour between coats. The wax should be buffed to a high gloss between coats and a minimum of three coats should be used. More coats are necessary for porous materials like plywood. Allow the wax to cure for 24 hours before the next step.

After the mold has been waxed, it must be treated with PVA (Poly Vinyl Alcohol). PVA is mistakenly referred to as mold release wax. It's not a wax at all. It's actually a plastic that is liquefied in alcohol. It forms a plastic film over the surface that facilitates the separation of the mold.

With the mold waxed and treated with a release agent, it's time to start laying up the fiberglass. Before we get into that subject, I want to spend a little time talking about a problem most of us have come across.

Print Through

One of the most common cosmetic problems with classic fiberglass boats is “print through”. This is when the weave of the fiberglass fabric begins to show through the gelcoat. It's caused by one, or both, of two factors.

The first factor is gelcoat shrinkage. Because gelcoat is simply thickened and pigmented polyester resin, it will continue to shrink and get thinner as time passes. When it does, any variations in the fiberglass lay up beneath it will become visible. This is obviously most easily seen on older boats, however boats only a few years old can exhibit the same quality. This is most often caused by the second factor, the lay up schedule.

As I said in the descriptions of the fiberglass fabrics, mat is the most waterproof. For this reason, boat builders would use it as the first layer behind the gelcoat. But there are other reasons why it is used as the first layer that are more cosmetic than structural in nature.

Mat is fairly thick, that's why it acts as a good binder between layers of cloth or roving. The woven fabrics nestle into the mat like it's a memory foam pillow. The result is, when mat is used as the first layer in a fiberglass lay up, it hides the wicker-like profile of the fabric behind it.

For decades, builders would spray the mold with gelcoat, then begin their fiberglass lay up schedule with one layer of mat and then alternate roving-mat-roving-mat... That first layer of mat was not enough to completely absorb and hide the weave of the roving, thus leading to print-through. It has since become common practice in the world of fiberglass boat building to use two layers of mat before laying in the first layer of roving (or cloth).




Sidenote: As always, safety should be your primary concern when working with Volatile Organic Compounds (VOCs). Always protect yourself. Wear nitrile gloves, (not latex), use a respirator, (not a dust mask), with fresh cartridges. Wear eye protection (MEKP will blind you if it gets in your eyes), and never use acetone (or any solvent) on your skin, just soap and water. Neoprene sleeves and bibs are available from most fiberglass suppliers and I do recommend them.


The time has now come for us to build our battery box. By this point, I assume the reader has a full understanding of the difference between a plug and a mold. Any further differences between the two in the lay-up phase should be self evident. To simplify the rest of this section, I'm going to assume that we are using a mold and not a plug. Since boats are built using molds, I think this is the logical point of view from which to write.

First and foremost, we need to talk a little more about gelcoat. Subsequent sections of this book will go into much greater detail on the subject, but for now, a cursory understanding of the product is vital. Forgive me if some of this is redundant, but I believe it's worth going over again.

Gelcoat, as mentioned earlier, is polyester resin heavily thickened with pigments. This is the color you see when you're looking at a boat. It is also typically air-inhibited. This means that the gelcoat, when exposed to air, will not fully cure.

Gelcoat is catalyzed with MEKP at about 2-3% by volume. More catalyst will cause it to cure faster. Because MEKP is a very thin liquid and gelcoat is much thicker, it takes a lot of stirring to ensure it is mixed completely. And because MEKP is clear when in liquid form, you won't be able to tell by looking at it when it's thoroughly mixed.

My recommendation is to use the low end of the scale for adding catalyst (2%) in order to give you a longer pot life, and spend a ridiculous amount of time and energy mixing it up. Spending an extra ten minutes stirring the gelcoat is a lot less time consuming than spending the next three days dealing with a finished product that has multiple spots of uncured gelcoat needing repair.

The reason gelcoat is air inhibited is to help the boat builder (or battery box builder in this case). As the gelcoat begins to cure, or “kicks” the surface (the side exposed to the air) will remain tacky while the side against the mold will harden completely. This is meant to give you a good mating surface in which to lay your first layer of fiberglass.


Before doing anything to the ready mold, you should pre-cut all of your fiberglass fabric. Make each piece a little bigger than you need, you'll be able to trim off the excess neatly while the fiberglass is still “green”. Stack your layers of fiberglass in the order that you'll be using them. Your first two layers will be 1.5 oz mat, followed by either a layer of cloth or a layer of woven roving depending on how fast you want to build thickness and how strong you want your battery box to be. My recommendation for a lay up schedule for a typical battery box would be something like this:

1 1/2 oz Mat

1 1/2 oz Mat

3.2 oz Cloth

1 1/2 oz Mat

3.2 oz Cloth

1 1/2 oz Mat


Although the lay up schedule can be as thick or as thin as you'd like, the important thing to remember is: Any two layers of fabric, must be separated by a layer of mat, and laminations must always start and end with a layer of mat.


With the mold waxed and ready, the first step is to spray (or brush) gelcoat into the mold. The goal is an even coating that is between 30 and 40 mils thick (about 1 mm). It's going to shrink a little as it cures which will bring it down to about 20-30 mils, and that's just about perfect. If there isn't enough gelcoat, then the glass fibers can show through. If there's too much, it gets brittle and can chip easily.

At 2% catalyst and 70 degree ambient temperatures, the gelcoat should take an hour or two for it to kick. When it kicks, you'll be able to press a finger onto the gelcoat without pressing though it. It will still be sticky to the touch, but not a liquid anymore.

The first two layers of fiberglass should be 1.5 oz mat. Use the Resin to Fiberglass ratios listed earlier to pre-measure the resin and catalyze it at 2%, just like the gelcoat. Resin, (and gelcoat), is exothermic. This means that it will generate heat as it cures. The faster it cures, the more heat it will generate. If you mix it too “hot” (too much catalyst) it will not only weaken the overall structure, but it can get so hot that it will actually catch fire. Additionally, mixing it with less catalyst will allow you to build up more layers at one time without having to wait for it to cool down.

Only mix up enough resin for the first three layers. The more resin you have sitting in a mixing pot, the faster it's going to begin to cure. Mix up small batches, that way you're not racing to get it laid up.

The next step is to thoroughly wet out the the fist layer of mat. Do this on a table covered with waxed paper or a similar work station that won't be ruined in the process. Don't do it on cardboard or another porous surface that will absorb your resin and screw up your glass to resin ratio. Then wet the whole surface of the mold with resin.

When the mat is fully wetted out and the mold is coated with resin, you can no lay in the first layer. Use a disposable chip brush to dab it in place making sure you get 100% contact with the gelcoat. Spend some time here making sure there are no voids or trapped air. When the contact is complete, you can wet out the second layer of mat and lay it in place.

The third layer will be either cloth or roving. Fully wet it out, just like with the mat, and lay it in place. Make sure you have 100% contact.

The next step is to take a resin roller, (this is a tool that will be available where ever you buy your fiberglass supplies. It looks like a standard J roller, but instead of having a perfectly smooth roller surface, it is serrated), and, using heavy pressure, roll out the entire surface of the layers until there are no air bubbles or voids. Keep rolling for a few minutes. This will compress the layers and ensure a strong lamination.

You never want to use a resin roller on a wet layer of mat. The loose fibers in mat will get caught in the roller and turn it into a rats nest of partially cured fibers, ruining the roller. Only use it on layers of cloth, roving, or other woven fabrics. When finished, soak the working end of the roller in a bucket of acetone until it's needed again.

Replace your gloves, (they are probably a mess by now anyway). Using a fresh mixing pot, mix up a fresh batch of resin for the next two layers. Brush a coat of resin on the laminated surface. Wet out the next two layers, (alternating mat and cloth), lay them in, then roll them out again. Repeat this procedure until the lamination has reached the desired thickness.

For a smoother inside finish, use two layers of mat after the last layer of cloth and dab them into complete contact using a chip brush.

Allow as much time as possible, (up to a week, but a minimum of two days), before removing the mold. Even with coats of wax and the release agent, this is not going to be easy. Enlisting the aid of some soft-wood wedges and a rubber mallet will help tremendously, but this is not without its dangers. Fiberglass and gelcoat that has not reached its full cure will be somewhat soft and easily damaged. Go slowly and work your way around the perimeter of the mold.


Side note: If you've decided to use the option of applying the gelcoat after the mold as described earlier, then do everything exactly the same starting from the first layer of mat. Once the piece has been removed from the mold, wash it completely with soap and water, sand the exterior surface with 80 grit sandpaper, solvent wipe it clean with denatured alcohol, and spray on the gelcoat.

If the gelcoat is air inhibited, wait for it to kick, the spray several coats of PVA over the surface to seal out the air.

The gelcoat will still need to be wet sanded, compounded, and polished for a finished look. (See article on gelcoat restoration).


Fiberglass Repair



This is the section that propbably should have been at the top. Very few fifty year old boats have lived their lives without sustaining some damage. It can be as small as a nick or gouge or as large as a completely stove in hull. This section will teach you the proper method for repairing that damage.

Most of the elements of fiberglass lay up are the same whether you're fabricating a new piece or repairing an existing piece. The lay up schedule is the same. The resin ratios are the same. The cure times, the alternating fabrics, the process for wetting out are all the same. The differences lie in how to attack the problem.

Even though many of the repairs will be accomplished exactly the same as each other, there seems to be a disconnect between certain kinds of problems and how they are repaired by the at home boat restorer.

For some reason, a Do-It-Youselfer often sees the task of filling in a hole where a piece of hardware no longer exists in a completely different light than a hole caused by an unintended impact with a dock. Even though the repair procedure for these two situations is identical, I will often see panic in the eyes and hear urgency in the voice of the guy who brings me his boat for repair because he hit something, but then get a sense of “ah, no big deal” when talking about the old holes in his transom where old hardware was through-bolted.

This never ceases to confuse me, but since I can't change how people view the world, I'm going to accept it and write about repairing it in two different sections.

Identifying Damage

The first step in repairing damaged fiberglass is assessing the extent of the damage. Sometimes it's easy to identify, (a hole is a hole), but often, the damage will extend further than what is visible. Other times, damage can go unnoticed because the visual cues are misinterpreted to be from another root cause. Usually, this is a misinterpretation of cracks.


Crazing is the result of UV damage. The ultra-violet light breaks down the binder in the resin used in gelcoat. Unreacted styrene evaporates leaving behind only the pigments used to color the gelcoat (the chalkiness of unprotected gelcoat is simply pigment that is no longer bound by resin). Left unattended, the whole thickness of the gelcoat becomes a house of cards. It is no longer structurally stable and will, either from expansion and contraction caused by temperature variations, or by the flexing of the boat underway, begin to crack across the whole surface. It looks like alligator skin. The cracks are uniform across the exposed surface, not concentrated in one area like damage from an impact.

Most people think of crazing as a cosmetic problem that requires only cosmetic solutions. What most people fail to take into account is that the resin used in gelcoat is the same resin used in fiberglass. The UV rays don't just destroy the gelcoat, then hit that first layer of fiberglass and call it a day. In extreme cases, the fiberglass itself will begin to craze. You won't know it until the gelcoat is removed and the fiberglass is visually inspected. If your boat is at this stage, it is very likely to be beyond what is financially reasonable to repair. I don't want to get distracted and move into an area of the book that will be covered in detail later, but suffice it to say, cosmetic problems have structural ramifications. Keeping your boat in good cosmetic condition is an element of safety, not vanity.

The only way to repair crazed fiberglass is to grind off all of the dead gelcoat and the damaged fiberglass behind it until you are down to clean glass. Then starting with a layer of mat, alternate laminating cloth and mat until the full thickness has been restored. The final layer should, as always, be a layer of mat. When all that has been done, you can fill and fair the surface and topcoat it, either with a marine paint system or by going through the unnecessarily arduous task of re-gelcoating a boat.

What will not work is the process of filling the cracks with epoxy or an epoxy primer since the whole thickness of the gelcoat has already been compromised. The cracks are simply the fracture points from expansion or flex, filling them will not stabilize the damaged gelcoat and fiberglass that hasn't cracked. It may appear to be a solution because the cracks are gone, but, like a house with a bad foundation, fixing the roof won't solve the problem.

Impact damage

Damage cause by impact, either by hitting something or dropping something, is easy to identify. Impacts will have an epicenter. The cracks will form concentric rings emanating out from the point of contact. Because gelcoat is unreinforced resin, it is more brittle than the fiberglass behind it. So the outermost rings (cracks) will likely be contained to gelcoat damage. However, to ensure a complete repair, each crack will need to be ground down to bare glass and reinforced. Missing a single ring is to compromise the entire structure within.


Sometimes easy to see, such as when the fiberglass tabbing around a bulkhead begins to separate from the hull, this kind of damage is the most dangerous sea. More often than not, the delamination will be either invisible or hidden from view, as in the case of delaminated stringers below the floor or between laid up layers of fiberglass.

A boat is a series of systems all tied together to make a vessel greater than the sum of its parts. A hull is useless without a keel, a stringer is useless without a hull, a floor is useless without a stringer, a bulkhead is useless without a floor, a deck is useless without a bulkhead. Obviously, every boat is different, but you get the idea. The structural systems of a boat are all tied together. When one part of that system begins to fail, the others will follow suit. Delamination is the quickest route to system failure and is often a silent killer. That's why it's essential to use good practices when laying up fiberglass.

Delamination grows from small voids in the laying up process. As a boat moves through the water, it is constantly flexing. If you've ever tried to remove a sticker from its paper backing, you know that getting that first corner to lift off is sometimes a pain in the ass. We usually hand it to our wives or someone else with long fingernails and say, “get this started for me, will you, Hon?” But once that first corner is lifted off the paper, removing the sticker is easy. That small void in the fiberglass lay up is the corner of the sticker. The flexing of the boat in the water is your wife's fingernail.

Checking for voids and delamination requires a small ballpein hammer. Tapping the fiberglass lightly will reveal a distinctive hollow sound where there is a void. Small voids can be ground out and filled with a fiberglass mash (kitty hair) while larger areas of delamination will need to be relaminated.

Osmotic Blisters (Boat Pox)

I really didn't want to include this section in the book for several reasons. First, classic fiberglass boats are nearly always trailered boats that don't live in the water for extended periods and thus, are not generally affected by blisters. Second, blisters are widely misunderstood, from the cause to the “cure”.

Still, to write a large piece about fiberglass damage to boats and not include osmotic blistering would be a glaring omission. So, this will be the first and last of what I have to say on the subject.

Osmotic blisters are caused by moisture penetrating the porous gelcoat, dissolving the water-soluble materials in the first layers of fiberglass laminate, and the chemically reacting with those materials to form molecules that are larger than the original water molecules that then become trapped behind the gelcoat. This results in a bubble behind the gelcoat.

That's it. In a nutshell, that's osmotic blistering. If you do an internet search on the subject, you will find page after page of ridiculously over-complicated explanations designed to instill fear in the reader and mask their own confusion on the subject. Many times, these descriptions are written by boat yards that would love to “repair” your blisters.

The “repair” can be as simple as grinding and filling small blisters, then adding an epoxy barrier coat, or as invasive as “peeling” off the gelcoat entirely, allowing the hull to dry out for weeks or months, laminating on a new layer of mat with vinylester resin, fairing the hull, and then barrier coating.

In the end, how you use your boat will be the determining factor to whether or not they will return. No blister repair is permanent. Some boats are more prone to blistering due to their use of cheaper Orthophthallic resins, but all are susceptible. Blisters are largely cosmetic damage (though extreme cases can lead to delamination), and the industry-wide focus on their repair is due to a combination of misunderstanding of the potential damage from buyers and sellers, and the cash cow it's been for boat yards. Thankfully, boats that don't spend 100% of their time in the ocean will generally not have to deal with this.

Severely Damaged Fiberglass and Other Reasons to Repair Fiberglass

This is an all encompassing term. Severe damage might be a tree falling on the boat. It might be a gaping hole caused by a collision. It can be caused by a storm, and it can be caused by a single moment of inattention. There are any number of reasons for catastrophic damage and the result always looks like something unsalvageable. This is the irony of fiberglass. Something as simple as crazing, which doesn't really even look like damage, can kill a boat, but severe damage caused by a falling tree nearly bisecting a boat is actually a fairly straight forward repair that, with the right tools and a willingness to itch for a while, can be done by just about anyone.

There are many ways to hurt a boat, and many other reasons to repair fiberglass. Hardware removal was mentioned earlier, routine nicks and dings are another... But these other forms of damage are usually easy to identify and the repairs are straight forward.

Small Repairs

Small repairs consist of dings and gouges that have penetrated the gelcoat and have affected the fiberglass substrate, but does not go all the way through the fiberglass. For information about repairing dings and gouges that have not affected the fiberglass, please refer to the Surface Prep stage of Painting a Boat.

While larger repairs involve building up new layers of fiberglass to retain structural integrity, small nicks and gouges can be repaired using fiberglass mash (or mush, or mish-mash, or kitty hair...they're all the same thing). It is nothing more than chopped strand mat added to resin until the consistency is similar to a sort of hairy putty. You can buy cans of it under various brand names, but they're all pretty much the same thing (Avoid automotive putties, resins, and fillers. These are usually made with orthophthallic resins and are not intended for marine environments). Or you can make your own in about two minutes, just keep adding mat until it's reached the desired consistency.

It should be catalyzed with a cream hardener are opposed to a liquid hardener. Both are just MEKP, but the cream hardener has color added to it that will help you tell when it is thoroughly mixed.

To prep the area, you'll need to grind back the damage until you get to clean smooth fiberglass all the way around. Grind it back at an angle, so the taper will provide some surface area for the mash to adhere to. Blow or vacuum out the dust and wipe the area clean with solvent. I prefer denatured alcohol, but acetone will work fine. The goal is a clean and smooth divot in the glass.

Mask off the area around the repair at least two inches all the way around. Getting cured structural filler off of gelcoat is not easy and, because gelcoat is softer than fiberglass, trying to sand it off will invariably alter the profile of the surface around the repair.

Press the mash into the repair. Don't drag it. Mash will compress just like fiberglass laminates, the goal is to get as much mash into the repair as possible. Be as neat as possible. You want to minimize the sanding as much as possible.

Side note: Here's a trick I use. When the repair has been filled and the resin has kicked, but not fully cured yet, I take a brand new razor blade and carefully slice off any mash that is proud of the surface of the gelcoat. You must be careful not to use too much pressure, it should slice easily. If there is too much resistance, then you can weaken the bond to the repair by “tugging” it out.

Allow the repair to fully cure, then sand off any excess flush with the rest of the laminate. You can then either fill and fair the surface in preparation for paint or repair the gelcoat in that area. An important note; you can use any resin of your choice to make mash, but if you use epoxy, you wont be able to repair the gelcoat since polyester doesn't adhere well to cured epoxy.

Larger repairs

Damaged areas that penetrate the laminate must be ground back and new layers laminated in place. Obviously, this gets a little trickier than simply applying mash.

The first step is to identify the thickness of the laminate. You need to grind back into stable fiberglass at a ten to one ratio. This means if you are repairing an area that is made of ¼” fiberglass, you need to grind back 2 ½” (10 quarters of an inch). If it's ½” thick, grind back 5”.

When you get to stage of adding new layers of fiberglass, you're going to start with the smallest layer at the bottom and build up, with each layer ½” bigger than the previous, until the whole suface is covered and flush with the surrounding fiberglass. This ensures that each layer of fiberglass you lay in will have direct surface contact with the boat.

The surface prep is the same as with smaller repairs, and the lay up schedule and technique is the same as described in the mold making section, so that information has already been covered. But there are still some factors to discuss.

The first factor is to decide which side of the damage to grind back, the outside or the inside. The answer is often couter-intuitive. If you're patching a hole in the bottom of the hull, most people would mistakenly assume they should grind back the fiberglass from the inside of the boat in order to preserve the exterior finish as much as is possible. In fact, the opposite is true.

When you grind back fiberglass to repair laminate, what you're essentially making is a tapered fiberglass plug that's bonded to the surrounding area. Because the plug is tapered, it will be much stronger when natural forces are pushing that plug into a hole rather than trying to pop it out of the hole. When a boat is in the water, the natural force of the sea is pushing against the bottom of the hull. If the repair is done from the inside of the boat, then the water is trying to pop the plug out of the hole. If the repair is done from the outside of the boat, the natural force of the water is trying to force the plug into the hole. As you can see, the repair done from the exterior of the hull will be considerably stronger than it's alternative. Yes, it will be a more involved task to make it cosmetically finished, but very few boats look good from the bottom of the ocean.

When repairing fiberglass, you need to understand this principle. Decide which direction the natural forces will press upon the repair, then grind back that side. If the forces are equal, such as on a bulkhead, then you can let the cosmetic aspect sway you one way or the other.

Another factor is the use of temporary molds. When laminating fiberglass, it is necessary for there to be resistance from the back side of the repair. This resistance is what allows you to compress the layers of glass to each other. For small laminations such as filling a one or two inch hole, a block of wood wrapped in saran wrap will do the trick. As repairs get larger and shapes get more complex, more of your time will be spent making sure you have an effective and sturdy temporary mold behind the repair.

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