Gelcoats

Marine gelcoat, and the one decision that ruins most first jobs

Gelcoat is pigmented polyester resin made to be the surface rather than the structure. It is what gives a hull its gloss and its UV protection, and it goes into a mold first, before any glass. Quart through 5 gallon, every size shipped with MEKP.

Waxed or unwaxed. Get this one right

Gelcoat is air inhibited: exposed to air, the surface will not fully cure and stays tacky. Waxed gelcoat has a surfacing agent blended in that floats to the top and seals it, so it cures hard and sandable. Unwaxed gelcoat stays tacky on purpose so the next layer bonds to it.

So: waxed for a final coat in open air, like a repair on a deck or a topside patch. Unwaxed when you are laying glass over it, which is every gelcoat sprayed into a mold. Using waxed gelcoat in a mold is the classic mistake, and it delaminates. If you have unwaxed gelcoat and need a hard finish, add surfacing wax to the last coat yourself.

White, neutral, clear and tooling

Pure white is the stock hull colour and the one most repairs need. Neutral is an untinted base for colour matching an aged hull, which almost never matches a tin of white straight off the shelf. Clear goes over graphics and veneers. Orange and black tooling gelcoat are harder and more heat resistant than hull gelcoat, built to survive being demolded dozens of times, and the high contrast colours make pinholes and scratches visible on a mold surface.

Spraying, brushing and thinning

Gelcoat sprays at roughly 15 to 20 mil in two coats and brushes heavier. It is too viscous for most guns as supplied, so it is thinned with styrene, not acetone, which flashes off and leaves the film short. Hold thinning to about 5 percent or the film loses build and gloss.

Catalyst and coverage

Gelcoat takes about 2 percent MEKP, noticeably more than laminating resin, and the window is narrower. The gelcoat calculator sizes the job by area, coats and mil thickness, and gives the catalyst dose for your shop temperature.