Gelcoats

Waxed gelcoat has an additive that rises to the surface as it cures, sealing out air so the coat hardens tack-free and is ready to sand. Unwaxed (sanding) gelcoat has no additive, so it stays tacky until you add surfacing wax to the final coat or cover it with PVA. Use unwaxed when you plan to lay more coats, and use waxed for the last coat.

About 2% MEKP by volume at 77°F, which is about 19 cc per quart or 76 cc (about 2.5 fl oz) per gallon. Use a little more in a cold shop and a little less in heat, but stay between 1.5% and 2.5%. Measure with a graduated dispenser, since drops are not a reliable measure.

At about 20 mil wet, one gallon of gelcoat covers about 80 square feet before losses. Allowing for overspray, plan about 60 square feet per gallon (about 15 per quart) when spraying, or about 70 square feet per gallon when brushing or rolling. Tooling and heavy color coats go thicker. The gelcoat calculator works it out for your job.

Yes. For small repairs you can brush or roll gelcoat, though spraying gives the smoothest finish. Thin it about 5% to 10% with styrene so it flows out and self-levels, catalyze with MEKP, and add surfacing wax to the last coat. Work in thin coats to avoid runs, and plan to sand and buff afterward since some brush marks are normal.

It is not recommended. Polyester gelcoat bonds well to polyester and vinyl ester laminates but adheres poorly to epoxy. If you must gelcoat over epoxy, let the epoxy fully cure, wash off any amine blush, sand it thoroughly, and expect a weaker bond. For epoxy repairs, a two-part polyurethane topcoat is usually the better finish.

Tooling gelcoat is a harder, more heat-resistant, high-gloss formula made for building fiberglass molds, not finished parts. Backed by tooling resin, it gives a mirror mold surface with excellent release and holds up to repeated part pulls. Orange and black are common because the strong color makes pinholes and thin spots easy to see. Standard gelcoat is for the finished part's exterior.

Clean the area with acetone, then sand or grind the damage to a slight V so fresh gelcoat can key in. Mask around it, mix gelcoat with the right amount of MEKP, and add surfacing wax to the final coat so it cures tack-free. Apply slightly proud of the surface, let it cure, then wet-sand from coarse to fine and buff to a gloss. Match the color first with pigment on larger or visible repairs.