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.

Start with a neutral or white base gelcoat and add polyester-compatible pigments a little at a time, testing on a scrap or hidden spot until the cured color matches. Cure shifts the shade slightly, so judge the match after it kicks, not while wet. Older hulls fade, so you may need to tint a little off the original to blend. For a solid white hull, a pure white gelcoat often matches with no tinting.

Clean and sand the surface, then catalyze the gelcoat with MEKP and lay it down in two to three coats. Spray it at about 18 to 20 mils wet, or brush and roll thinner coats, letting each one flash before the next. Because polyester gelcoat is air-inhibited, the final coat needs surfacing wax (or a PVA film) to cure tack-free so you can sand and buff it.

Catalyze polyester gelcoat at about 1% to 2% MEKP by volume, with 1.25% a good starting point near 70°F. That works out to roughly 1.5 to 2.5 ounces of MEKP per gallon of gelcoat. Use a little less in hot weather and more when it is cold, and always follow the ratio printed on your MEKP. Do not exceed about 3%, which can crack or discolor the coat.

Only if you are using unwaxed (sanding) gelcoat. Unwaxed gelcoat stays tacky on the surface because air stops polyester from curing, so you add surfacing wax to the final coat, or spray PVA over it, to seal out air and cure it hard for sanding. Gelcoat sold with wax already contains it and cures tack-free on its own.

A tacky surface almost always means air-inhibited cure, not too little catalyst. Unwaxed gelcoat will not harden on top until you add surfacing wax to the final coat or seal it with PVA. Other causes are too little MEKP, temperatures below about 60°F, or high humidity. Confirm your catalyst ratio, warm the shop, and wax or PVA the last coat.