Gel Coat Repair for Mortals

Try this in a composite kayak and you might be needing some gel coat repairs. Photo: John Carmody

This article was originally published in Sea Kayaker Magazine, February, 2006.

[Edits and updates in brackets]

Gel-coat scratches in a fiberglass kayak are a fact of life. If you paddle often and paddle hard, you’re bound to scuff up the bottom a bit. This is good. It means that you’re spending time out on the water enjoying your boat. If you can learn to ignore superficial scratches in the gel coat, your life will be better.

However, there comes a time when even the most hardened boat abuser starts to think about fixing things up a bit. Extensive spiderweb cracks, chunks of missing gel or wear through to the glass laminate are worth taking a look at.

The good news is that working with gel coat is not terribly difficult. Even us mortals can get a polished gel-coat repair. The basic procedure for making a repair has five parts: internal hull patching, surface prep, gel-coat application, sanding and finishing. If you follow this progression and are patient, you’ll get good results.

Gel Coat Damage
Severe Damage: This gel coat damage is the result of a crushing force to the hull. A lasting repair will require internal fiberglass patching.

Causes of Damage
Before you begin your repair, it makes sense to think about what causes gel-coat damage in the first place. There are three primary sources: impacts, abrasions and repeated stress. Impacts may damage gel coat by deforming the hull enough that the gel exceeds its ability to flex. The resulting fracture in the gel coat will often look like a star or a spiderweb. With a heavy impact, it’s possible to break a chunk of gel coat off of the hull, leaving a deep gouge or even a hole in the boat.

Abrasion often happens to the keel of a kayak at the stem and stern. Fiberglass and gel coat are quite abrasion resistant, however, dragging over rocks or coarse sand can wear through the gel. Heavy abrasion to the keel is often repaired by the application of a full keel strip to the kayak. Minor abrasions may be fixed, as outlined in this article.

Cracks from repeated stress are also common. This is particularly true in boats that have rigid bulkheads rather than foam. The bulkhead in this case creates a hinge point or “stress riser” against which the hull flexes. Repeated flexing over time may soften the laminate in this area and give rise to cracks that run across the hull at the bulkhead.

It’s not safe to assume that the fiberglass beneath the cracks is undamaged. If the boat was impacted hard enough to crack the gel coat, or if it’s been flexing at the bulkheads, the underlying laminate may be weakened. Even if there’s no visible damage or leakage, the fiberglass may have been stressed enough to lose some of its integrity and strength. If this is the case, you’ll need to apply a patch to the inside of the hull to prevent the cracks from returning.

Materials Compatibility
The vast majority of fiberglass kayaks are made with polyester resins. Some Kevlar kayaks are laminated with vinylester or epoxy resins. The techniques outlined in this article are intended to be used on kayaks that are laminated with polyester or vinylester resin and polyester gel coat. If you have a kayak that’s laminated with epoxy, you won’t be able use the materials listed here (polyester resin and gel coat will not adhere to epoxy). The greatest likelihood is that your kayak is laminated with polyester resin. If you’re in doubt, contact the manufacturer.

Health Precautions
Whenever you’re working with fiberglass, make sure that you take precautions against dust, fumes and chemicals. Wear a dust mask while sanding and a respirator that protects you against chemical vapors. Nitrile gloves offer good protection against the chemicals that you’ll be using, most of which can be absorbed through the skin. Work in a well-ventilated area.

Interior Fiberglass Patching
You only have to apply a fiberglass patch if the hull is soft in the area of the gel-coat damage. If not, skip this step and proceed directly to surface preparation.

[Acetone can be used for all surface prep and to remove wax residue between coats.]

Internal Fiberglass Patch
Finished Internal Patch: For this cured and sanded patch, woven glass cloth was placed over the glass mat in this repair. The resin was not pigmented, thus the patch has cured a light brown color.

For patching, you’ll need fiberglass mat, waxed polyester resin, hardener and sandpaper. Glass mat is an unwoven fiberglass cloth that looks like blotter paper or felt. It’s commonly available in an ounce-and-a-half weight and works well for patches because it conforms to curves and absorbs enough resin to be stiff. Fiberglass cloth has a higher tensile strength than glass mat and is used in the construction of light, strong kayaks. However, glass cloth doesn’t hold as much resin and is harder to conform to tight spots.

Waxed resin contains paraffin wax as a surfacing agent. Unwaxed resin will dry with a sticky surface and is used for laminating multiple layers of glass cloth. Waxed resin will dry with a hard, waxy finish. If you need to put another layer of cloth or resin onto this surface after it has cured, you must sand the wax off and wipe with a solvent-like acetone or denatured alcohol.

You’ll need a workspace temperature of above 65˚F. Polyester resin is sensitive to temperature, and both the kayak and the workspace must be warm for your repairs to cure. The ratios for catalyzing resin are usually calculated at 77˚F. Temperatures cooler than this will slow the hardening of your repair. Temperatures warmer than 77˚ will speed hardening and shorten the working time or “pot life” for the resin. A large batch will cure more quickly than a small one.

With 60- or 100-grit paper, sand the area inside the hull that is to receive the patch. When you’ve thoroughly roughed the surface, wipe down the sanded area with denatured alcohol [or acetone]. Next, cut a patch of glass cloth about a half inch larger than the damaged area. Catalyze the resin to the ratio recommended by the manufacturer. This ratio is typically 15 drops of hardener to one ounce of resin. Place the patch onto a piece of scrap cardboard or wax paper and apply catalyzed resin with a brush (an inexpensive chip brush works well) until the fiberglass cloth becomes translucent.

Wetting Out Glass Mat
Wetting Out Glass Mat: The piece of glass mat on the right has been wetted out. Note that the larger piece will cover the smaller one inside the hull

Paint a thin layer of resin onto the hull where you’ve sanded. Remove the “wetted out” patch from the cardboard and apply it over this resin. Using the same brush, paint more resin onto the patch until the surface is smooth and there are no visible air bubbles. Allow the patch to cure and sand it to ensure that there are no sharp edges inside the hull. Wipe with alcohol [or acetone] as before, and catalyze a bit more resin. Paint a thin coat of resin over the patch to complete the repair.

For a simple gel-coat crack where there’s no major structural damage, you usually don’t need more than one piece of ounce-and-a-half fiberglass mat as a patch. However, some people may choose to lay a piece of woven fiberglass cloth over the mat patch to more closely match the appearance of the inside of the hull. If you choose to do this, cut the woven cloth about a half-inch larger than the mat patch, and wet it out at the same time. Lay the wetted-out cloth over the mat. Finally, finish the repair with additional resin and sanding as outlined previously. It’s possible to pigment the resin in an effort to match the hull color. Left to its own devices, polyester will cure a translucent brown.

Exterior Surface Preparation
To get a quality gel-coat repair, you need to prep the damaged area properly. This means chipping away any loose material and clearing the gel coat away so that you’ll be applying new gel directly to a stable fiberglass substrate. I use a motor tool with an abrasive bit to do this work. You may follow individual cracks with a fine bit or rough out the area between closely spaced cracks.

Prepping Spiderweb Cracks
Prepping Spiderweb: Prepping gel damage using a high-speed motor tool. Make sure to grind down through the gel layer to reach the fiberglass substrate. You may follow the cracks or rough out a larger area.

Be careful not to remove the fiberglass laminate itself as you grind down through the gel coat. You’ll want to taper or “feather” the edge of the repair area into a shallow dish or “V” shape as you work. The next step is to sand with 150- to 220-grit paper. Using the sandpaper, continue to feather the edges of the prepped area. Work out onto the hull of the boat so that you’ve sanded a spot that’s roughly rectangular and extends at least an inch beyond the bare fiberglass.

If you have a Kevlar kayak, you’ll have trouble with prep because of the material’s tendency to “fuzz” when it’s sanded. On a Kevlar boat, it’s best to do all of your prep with a razor knife and to keep your sandpaper as far away from the substrate as possible. If you have some fuzzing, you can still complete the repair. Loose strands that are captured in hardened gel coat are more sand-able. Just be prepared for a less-than-perfect result.

Once you’ve finished sanding, wipe the surface of the repair with denatured alcohol [or acetone], making sure to remove all surface debris. Next, mask off the area around the repair with blue 3M masking tape. This tape is preferred, as it doesn’t leave a sticky residue when it’s removed. Make sure that your masking tape overlaps the edge of the sanded area, so that when you apply gel coat, it will only be applied to a sanded surface. If the repair is to be done on a surface that isn’t flat, you may need to create masking-tape “dams” to prevent drips. Creative use of masking tape will also help you build up thicker layers of gel coat to repair deep chips or gouges.

Masked and Ready
Masked Area: A large prepped area ready for gel-coat application. Note the use of a tape dam at right to prevent liquid gel coat from dripping down the hull.

Applying the Gel Coat
Most kayak hulls are white, and white gel coat is available at most marine supply stores. If you’re trying to repair a colored hull or deck, you’re often best off contacting the manufacturer of your kayak for pigment or gel coat. Some manufacturers will send you a small vial of pigment for free, which may then be mixed with clear gel coat to create the proper color. Other manufacturers sell small containers of colored gel. It’s safe to assume that, because of sun fading or darkening, your color will not match even if you get it directly from the manufacturer. Most repairs blend with time as the patch weathers.

First, the gel must be mixed with catalyst in the proper ratio. This ratio is specified by the manufacturer, but in practice, polyester gel coats are fairly tolerant of imprecise mixing. I typically use small waxed-paper cups for mixing. With one-quarter inch of gel coat in the bottom of one of these cups, I add about 15-20 drops of catalyst. This ratio is a bit “hotter” than recommended, but it works well.

Once you’ve mixed your gel coat, you have only a certain amount of time that you can work with it. As with the resin, gel-coat’s cure time will vary with the temperature of your workspace and the amount of catalyst. This also applies to the thickness of the gel coat that you apply to the hull. A super-thin layer of gel will not cure at all unless you add extra catalyst. Most of the time, ratios of catalyst for home use are measured in drops per ounce of resin. In cool temperatures, or with tiny batches, you need to add more drops. In these circumstances, I might add 25 drops to my cup of gel coat. The ratio of catalyst to resin should never exceed two percent. This is pretty difficult to measure in small batches, and in practice, finding the correct ratio often relies on experience and a bit of guesswork. Remember, polyester resin is forgiving—don’t worry.

Styrene and Wax
Wax and Styrene: Paraffin canning wax shaved into thin feathers is ready to be mixed with styrene for use as a surfacing agent.

Gel coats are designed to cure in an air-free environment. When a kayak is built, the gel-coat layer is put into the mold first and allowed to set a bit before the fiberglass is laid over it. When you do repairs, you are essentially working backwards and need to create a barrier to oxygen so that the gel will harden properly. The barrier can be created by taping a piece of waxed paper over the wet gel once you’ve applied it. However, this tends to put ripples into the surface of the patch. I prefer to add waxed styrene as a surfacing agent.

Waxed styrene can be hard to find in the U.S. I make my own by shaving tiny flakes of canning paraffin was (available at grocery stores) into straight styrene (supplied by many marine stores). Shave paraffin into the styrene and stir the mixture until the wax dissolves. If you add too much wax, the mixture will become cloudy. Add a bit more styrene if this happens. If you use waxed styrene, add about 10 percent to the gel-coat mix before you add the catalyst. If you can’t find styrene, you may use waxed polyester resin instead.

Mix the 25 percent resin with 75 percent gel coat. This technique works well but tends to darken the gel coat a bit. Consider adding pigment to the gel coat if you’re using waxed resin instead of styrene.

There are many ways to apply the gel coat to your kayak. I have seen people paint it on, scrape it on with a razor blade, dab it on from the end of a stirring stick or pour it into a masking-tape dam—all of these work. Brushes work well but may leave bristles in your work. Foam brushes are good for thin layers but may dissolve and disintegrate during application. Hobby brushes with fine bristles work particularly well for applying gel coat precisely to small areas.

Applying Gel Coat
Applying Gel with a Hobby Brush: Catalyzed gel coat and surfacing agent is applied to a prepped and masked area. Be sure to make your layer thick enough—gel coat will shrink as it dries.

Apply a first coat thick enough to fill the gouges and restore the contour of the hull once you’ve sanded. Keep in mind that the gel coat will shrink as it cures. Apply gel coat to the hull so that it covers the prepped area up to the masking tape completely. Once the gel coat has started to harden a little, peel away the masking tape to reveal a clean edge. If you wait too long, the gel coat will harden completely, and you’ll have to chip it away from the tape before you can sand.

Once the gel is cured, you can begin sanding. This process is fairly straightforward, but there are a few tricks.

Masking Removed
Cured Gel Coat with Masking Removed: This area is ready for wet sanding with 220 grit paper.

Wet-sanding the repair with water speeds cutting and keeps sandpaper from clogging. Wet-dry sandpaper is readily available in grades finer than 300 grit and can be found coarser if you look around. Ideally you’ll have wet-dry sandpaper in 320, 400, 600, 800 and 1000 grit. The 800-grit and finer papers can be found at marine-supply stores. Unless your repair is huge, you’ll only need one sheet of paper per grade. [400, 600 and 1000 are sufficient].

It helps to use a sanding block of some sort. Rectangular chunks of minicell foam work extremely well. A minicell block will conform to the curved shape of the hull better than one made of wood or rubber.

Sanding with 220 Grit
Sanding with 220 Grit: Use a foam sanding block. Take care not to remove too much material early in the process.

Start with 100 or 150 grit to remove material. Do the rough shaping with this paper and switch to 220 to remove any deep scratches. Once you’re satisfied with the surface, move to 320. Each progressively finer paper will remove the scratches from the previous grade. Moving through all the grades doesn’t take long, so don’t start with the finest grade right away. If you jump from 320 grit to 1000 grit, you’ll eventually remove the scratches, but it’ll take forever and use up a lot of sandpaper. Keep in mind that even fine sandpaper will remove some material from the hull. Try to anticipate this as you sand, so that your repair finishes flush with the hull.

Use plenty of water. When you think that you’re finished with one grade of paper, wipe the sanding residue off of the hull with a rag and look at the repair from an angle to get a better view. This will let you get a better look at the finish. You’re looking for obvious scratches and for a uniform texture that indicates that the area has been thoroughly sanded.

Additional Application of Gel
Additional Application of Gel Coat: A thin layer of gel coat may be applied after sanding with 220 grit paper. A hobby brush works well. Remember that thinner layers require more catalyst to properly cure.

Before you move beyond 320 grit paper, take a look at the repair and decide if you’re happy with it. This is the time to apply another layer of gel coat if you need to. This coat should be smooth and thin so that it can be sanded and faired with 400-grit paper before you move on. If you need to add another layer, lightly sand with 220 grit, mask off the repair and repeat the application of gel coat.

[These days I dry sand with 220 and wetsand with 400, 600 and 1000. The intermediate grades aren’t necessary].

If you’ve done your work in the sanding department, the next part will be easy. You’ll be using a buffing bonnet on a drill or grinder to polish the hull. It’s difficult to apply enough “elbow grease” to shine past the 1000-grit paper that you used. Don’t use a car buffer. Most polishers used for cars are too gentle to bring out a deep shine in fiberglass. Using a high-speed polisher will let you buff the hull to a full shine and will blend away the last of the sanding marks.

Sand up to 1000 Grit
Sand up to 1000 Grit: Begin your finish sanding with 320 grit and move up through 1000 grit. Each progressively finer grade of paper will remove the scratches from the previous grade.

With the drill and bonnet, you’ll need to use a rubbing compound. There are plenty of rubbing compounds available, but my favorite is 3M Super Duty Rubbing Compound, which is available at marine-supply stores. 3M also makes a product called Finesse-It, which is intended to be used after their rubbing compound to increase the shine. Try the rubbing compound alone first. You can always buff again with the finer compound if you’re unsatisfied with your initial results.

Rubbing Compound and Polishing Bonnet
Rubbing Compound and Polishing Bonnet: Using a professional grade compound will improve your results considerably. This 3M compound and polishing bonnet were purchased at a marine-supply store.

Apply the compound to the hull and go to work with the buffer. Use steady pressure and move the bonnet over the area. Keep the surface of the hull moist. The rubbing compound will dry as you use it, so add more to the hull or dribble a little water on to keep things lubricated. You shouldn’t have to polish for too long before you have a mirror shine. Wipe away the compound from time to time and look at the hull from an angle. If all has gone well, it will be hard to tell the repaired section of the hull from the rest of the boat.

[If you do decide to polish with Finesse-It, make sure you flip over your polishing bonnet to use the other side. Or thoroughly clean it to remove all rubbing compound. If there’s rubbing compound residue on the bonnet you won’t be able to improve the shine with Finesse-It.]

Buffing Complete
Buffing Complete: Finished buffing will bring out a glossy shine but may reveal flaws in your work. Note the poor color match in this repair. The area at the bottom of the repair was not completely filled with gel coat during application.

Practice Makes (Mostly) Perfect
Your first attempt at gel-coat repair may take a little longer than an afternoon, but once you have the hang of it, it’s easy to do. With practice, your results will improve, and you’ll learn your own tricks. Once you’re confident with gel-coat repair, you’ll be less worried about using your boat to its full potential. Remember not to worry—nothing is forever in fiberglass.

Materials List

  • 3M blue masking tape
  • 3M Finesse-It polishing compound (optional)
  • 3M Super Duty Rubbing Compound
  • Acetone
  • Paraffin canning wax
  • Catalyst
  • Denatured alcohol
  • Drill or grinder with polishing bonnet
  • Fiberglass mat
  • Gel coat
  • Paper cups
  • Pigment
  • Sandpaper 60, 100, 220, 320, 400, 600, 800, 1000 grit [100, 220, 400, 600, 1000 are sufficient]
  • Stirring sticks
  • Styrene
  • Waxed polyester resin

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