Gelcoat is much tougher than paint. It can last for decades, if it’s properly maintained. And unlike some other coatings, neglected and abused gelcoat can often be brought back to life.

The story of gelcoat begins in the mid-1800s, with the invention of plastics. Swedish chemist Jöns Jacob Berzelius discovered that tartaric acid (found
in grapes, bananas and other fruits) reacted with glycerol (from soybeans, palm or tallow) to create a sticky resin. German chemist Daniel Vorländer developed an early unsaturated polyester, and then, in 1936, Carlton Ellis of DuPont was awarded a U.S. patent for polyester resin. This resin is the basis for modern gelcoat.

By the early 1940s, glass fiber saturated with plastic resins was being used to produce boat and airplane parts. California chemist John Wills then figured out how to cure the material at room temperature, ushering in the era of fiberglass boats we know today.

If they can figure out all of that, then modern-era boaters can learn how to properly maintain and repair gelcoat.

Initial Application

Fiberglass boats are typically made with a female mold. The highly faired and polished mold is coated in a release agent like wax. Then, catalyzed gelcoat is sprayed at a precise thickness.

After the gelcoat starts to harden, but is still tacky, the fiberglass and resin are added, making for a good chemical bond. Additional layers of fiberglass or core may be added, depending on the design. When the mold is removed, the gelcoat is glossy and fully cured, providing a beautiful, durable surface.

If gelcoat is sprayed into a mold before the fiberglass is laid, then the later layers stop air from reaching it. However, if plain gelcoat is coated on the surface outside the mold, then it will remain sticky and may take weeks to fully cure. This stickiness creates a good chemical bond if you want to overcoat with more gelcoat, but the last coat must exclude air prior to curing. A wax additive in the final coat of gelcoat can accomplish this. The gelcoat can also be sprayed with a water-soluble liquid called polyvinyl alcohol, or small repairs can be overlaid with a clear sheet of plastic release film.

There are benefits to each method, depending on the size of the repair and how complicated the shape is. And, you can buy gelcoat with or without wax added. If you are adding it separately, wax should be mixed at a ratio of 5 percent into the final coat only (a quarter-ounce per pint of gelcoat). Too much wax will result in undesirable results, including blemishes called fish-eyes. After adding the wax, you still need to add the appropriate amount of catalyzer.

Lead and styrene are the two most harmful agents in gelcoat. Don’t breathe gelcoat fumes or have it touch bare skin. Even once it’s dry, you don’t want to breathe the dust when you’re sanding or polishing it.

Also note that gelcoat will go bad in the can over time, so don’t buy more than you think you’ll use.

Making Repairs

Four main types of gelcoat damage require repairs: cracks, impacts or chafing (possibly with chipping), air pocket voids that open, and thinning that reveals the glass matrix below. Repairs for all of these problems are similar, with a few variations.

Cracks typically form in gelcoat that was applied too thick, or if there is twisting stress, causing more movement in the structure than the surface gelcoat coating can flex. Cracks are common at right-
angle interfaces of deck-to-cabin structures, especially if there is not enough support of the deck underneath. You may recall a spate of boat manufacturers in the 1990s that used thicker gelcoat as a selling point, until they realized it was causing an inordinate amount of spider cracks.

Properly repairing cracks requires grinding the length of the crack down to the glass with a die grinder or Dremel tool. You create a V- or U-shape groove, then clean the area with alcohol and fill the crack with polyester, vinylester or filler.

Vinylester fillers can be more resistant to water absorption. If the grooves are deep, a reinforced filler with added structural fiberglass will be stronger, but will also be harder to sand fair. In some cases, it makes sense to perform two (or more) fillings, with the first layer being reinforced, and with standard filler on the upper surface for easier sanding.

The fillers are a similar base to the gelcoat. They are a resin with more thickeners added. They still require a catalyst. In some cases, the catalyst can be purchased in different colors to make it easier to know when it has been mixed properly. If your topcoat is white gelcoat, then use a white filler to avoid show-through. Don’t mix fillers on cardboard; use a piece of plastic or metal. Cardboard can absorb the catalyst and cause curing issues.

If you’re repairing flat areas by applying one or more coats of gelcoat on top of your filler, then you need to first sand a relief into the existing gelcoat. Not too much, say half the thickness of a dime. This is an important step because when it’s time to wet sand the final coat, if you haven’t planned a relief and you try to sand the repair flat, you’ll have a hump or you’ll sand through your new gelcoat.

At some point, there may be too many cracks or too large an area of repair. Painting may become the only economical option. A repair at this point typically calls for aggressive grinding of most cracks, followed by filler and fairing. After this, brushing or spraying an epoxy barrier coat product (such as Interlux InterProtect, Pettit Protect or Sea Hawk Tuff Stuff) can stabilize the substrate prior to priming and painting.

We occasionally hear about someone applying gelcoat to an entire deck or hull. This typically isn’t cost-effective because gelcoat doesn’t flow like paint, and it always requires polishing after application. On a large or complicated surface, that can represent a huge amount of labor.

Color Matching

After you have made your repairs and faired the surface, you’ll need to match the gelcoat color. If your boat is new, then the builder or a company like Spectrum Color may be able to supply the original gelcoat color. Note that even after only a few years, gelcoat can fade; it may be necessary to tint the color from the can for a better match.

Pigments are made of minerals and metal oxides such as iron, titanium and chromium, as well as cobalt and ultramarine. Azo and quinacridone pigments, and carbon black, help to create all the colors of the rainbow. These pigments are concentrated in a gelcoat base for compatibility. They are potent. A little will go a long way.

The easiest way to match gelcoat colors is with a pigment kit. This will supply a basic range of colors that can be added to white or neutral gelcoat. Mixing coloring agents into neutral gelcoat creates darker, bolder colors, while a base of white gelcoat produces lighter, more-pastel colors.

Always make twice as big a batch as you think you’ll need. The uncatalyzed gelcoat will keep in a sealed container throughout the project, in case you need to add another coat or modify the color.

It can be a challenge to match a color. Just step into your local hardware store and look at all the variations of white paint chips. Whites can be broken down into cools and warms. Cools have blue and green tints, while warms have red and yellow tints. Gray-whites use black as a tint.

When mixing colors, always use uncatalyzed gelcoat. This lets you mix a little and dab it on the surface that you are trying to match. If it is not correct, you can wipe it off, add a little more pigment and try again.

Use the tiniest amount of pigment (fractions of a drop) for each try. It is better to creep up to the correct color. You can’t really undo a batch if you oversaturate it with too much color. Also, have plenty of clean stir sticks. You don’t want to contaminate your pigments.

Next, you’ll need to catalyze the gelcoat with methyl ethyl ketone peroxide before you apply it. The amount of catalyst can be adjusted for hotter or colder weather, and for the size of the batch. Working time is usually about 20 minutes. The ratio is 2 percent at 60 to 70 degrees Fahrenheit. This ratio can be adjusted if it is hotter or colder, but should not be more than 2.8 percent or less than 1.8 percent.

Graduated cups or syringes are a great way to make sure you are using the correct ratio. Remember that on the last coat, you will need to add wax or a plastic sheet, or spray with polyvinyl alcohol so the gelcoat will cure properly. If you use wax in an early coat, then you must sand the entire surface and de-wax it with alcohol before the next coat.

There are two ways to apply the gelcoat. Small repairs can be brushed, usually with disposable chip brushes, while larger repairs will come out nicer if they are sprayed. Brushing tends to leave brush-mark grooves. When you go to sand the grooves fair, if you are not careful, you can sand right through the gelcoat. Preval disposable sprayers can work well for small jobs, while larger jobs will benefit from a system such as the 3M Performance Spray Gun, powered by compressed air. The gelcoat will need to be thinned with styrene, usually 2 percent to 5 percent, so it will flow well through either system.

You may also discover that your first coat of gelcoat doesn’t match quite the way you expected. This is an opportunity to add a touch more pigment. You may also find that repairs on either side of the boat are a different color. Be aware that a repair may be almost invisible when finished, but given some time in the sun, it may start to show as the new gelcoat ages at a different rate from the old.

Once the gelcoat has cured, it is time to sand. Wet-sanding grits are typically from 220 to 1000 grit. Use the finest grit that will remove material at a reasonable pace. Rinse the sandpaper frequently to keep the grit clean. Flatten the area with a sanding block in circular motions.

Once most of the scratches are removed and the area is blended, you can use compound to finish the polish, either by hand or with a machine. Just be careful not to burn through the new gelcoat.

Protecting the Gelcoat

Even if you don’t have repairs, it is still wise to protect your gelcoat. Keep your boat in a shed or covered, as defense against the sun. If you are always moving or moor outside, then keep up with waxing. It’s like putting on sunscreen. Just like with a car, when the boat’s surface stops beading water, it is time to reapply wax.

If you can wipe across the gelcoat and its dust comes off on your hand, then the surface is oxidized. It will need compound to remove the oxidation.

Compound comes in different grits, from aggressive to light. Remember that polishing is removing gelcoat, so you want to use the least-aggressive compound that will get the job done in a reasonable amount of time. Large surface areas will benefit from a heavy-duty buffer and wool pad. Follow that process with a high-quality wax to seal and protect the surface. 

This article was originally published in the October 2024 issue.