Gelcoat Repair Techniques

Catalog Number 000-364


Many of the questions concerning the repair of fiberglass boats seem to revolve around the techniques used when applying polyester gelcoat to cover a repair made with WEST SYSTEMŽ epoxy. There are several steps to a successful repair. It is important that these steps are done in the proper order to assure a well matched repair color.

  1. One of the steps that is frequently left out is to apply a sealer coat of epoxy to the repair area. This is necessary to fill any porosity in the patch. Apply two or three very thin coats of epoxy, extending each coat slightly beyond the previous one. Warm the area with a heat lamp to speed the cure and to help the epoxy flow out nicely.

  2. When the sealer coat has cured, wash thoroughly with water and a 3M Scotch-brite pad to remove any amine blush. Any blush left on the surface may inhibit the cure of the gelcoat. This very important step is often forgotten, and the result is usually an unsatisfactory repair.

  3. Sand the epoxy coated repair area with progressively finer grits of sandpaper. Finish with 220 wet or dry.

  4. De-wax an area twice as large as the diameter of the repair. Apply tape around the perimeter of the de-waxed area. Use masking paper to protect the boat from overspray. If there is a molded bodyline or corner near the repair, you may want to extend the color patch to that point. The same is true for a painted or vinyl stripe.

  5. Sand the additional area out to the tape line with 320-grit paper. This will be the total area to be gelcoated.

  6. Determine the gelcoat batch size for the size of the repair--approximately 80 square feet per gallon of gelcoat (20 sq. ft. per qt.).

  7. Tint the batch of gelcoat to match the color of the boat. If the boat is fairly new and the manufacturer is still in business, you may be able to get gelcoat that will be a very close color match. If this is not an option, you will need to get the gelcoat from a FRP product supplier. Many marine distributors handle the gelcoats from various resin manufacturers. You will also need pigments to tint the base color to obtain a good match. These pigments are generally available from the gelcoat suppliers. Frequently, local repair yards that do fiberglass repairs will sell these products to the project user.

    There are several gelcoat additives that, when used properly, make matching a repair easier. These are clear, low-viscosity resins which are not air inhibited. These products are mixed with the pigmented gelcoat in place of a wax solution to provide a tack-free cured surface. They also provide the added benefit of thinning the gelcoat without changing the color of the cured patch the way acetone or styrene can. A few such products are Duratec Hi Gloss Clear product number 904-001; Fast Patch by American Colors; and Cook Paint and Varnish Company's number 970-X-900 Speed Patch Additive.

    Matching the repair color to the color of the boat is probably the hardest part of the entire job. You will find that most gelcoat colors change as they cure. As you tint the gelcoat to match, apply a small amount of uncatalyzed material to the sanded area surrounding the repair. Use your gloved finger to spread this into a sample the size of a quarter. Wait a couple of minutes for the solvents to flash off. Any color variation will be evident. If the color match is not acceptable, change the color by adding small amounts of tinting pigments. As you adjust the gelcoat color, try to think in terms of the basic colors, i.e.; the color needs to be blacker or redder or bluer or greener... This will help you identify which of the pigments to use. Use very small amounts of the pigments. You may even need to dilute the pigment with white gelcoat to weaken the effects. Apply an uncatalyzed smear with each change of color no matter how small. Leave each of these smears on the surface until you have the color as close to the boat color as you can get it. When you are satisfied with the color match, wipe away all of the test smears with acetone or lacquer thinner.

  8. Divide the batch into a 2/3 portion and a 1/3 portion. Our technique for using the patching additives is slightly different than the recommended procedure. Mix the patching additive with equal parts of the 2/3 portion of the matched gelcoat. Catalyze following the recommendations of the gelcoat supplier. Over or under catalyzation may prevent the product from reaching a proper cure. Apply this mixed gelcoat to the surface of the repair with a spray gun. Apply several light coats, feathering each one farther from the repair area. Allow the solvent to flash off between coats. You may need to apply five or six coats to hide the shadow of the repair. Most gelcoats colors will dry lighter in color if they are too thin. Most manufacturers recommend a total film thickness of 15 to 20 mils. A common mistake is to apply two or three heavy coats, causing solvent entrapment and improper curing. This may also have an effect on the color of the cured repair.

  9. Make another mixture with the 1/3 portion of gelcoat and patching additive. This time use about three parts of patching additive with one part gelcoat. Spray the first coat of this mixture over the previously sprayed area. Then apply two or three more coats of this mixture over the entire area. Extend each coat farther than the previous one, with the last coat out to the tape line. The semi-transparent film of lightly tinted clear patching additive will allow the original gelcoat color to show through around the perimeter of the patch. This will help blend any subtle color change into the original gelcoat.

  10. Once the gelcoat has reached a full cure, pull the tape and sand the tape line smooth. Start with 320-grit wet or dry paper and work up to 600-grit. Sand the surface of the repair if necessary.

  11. Buff the surface with a white, medium-cut rubbing compound. Take your time and do not overheat the surface while buffing. The excess heat can cause a stain which will be very difficult to remove. Finish with a fine compound and wax the area.

There are times when the repair will not match simply because the original gelcoat color has faded. You may need to polish the entire side of the boat to restore the original color.

Patience and experience are the two most important ingredients for a well matched color repair. The mixing of the color is guaranteed to teach patience. And since you may need to try more than once for a proper match, the experience comes quickly. So get to it.

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