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Once the fiberglass layup of a boat is fully cured, neither epoxy nor polyester resin will form a chemical bond. Most gelcoat is a polyester resin although epoxy gelcoats are available and the only possible bond to a gelcoat repair is a mechanical bond from a roughened surface. The only possible bond in any fiberglass repair is a mechanical bond to a roughened surface. It is a fact that epoxy resin is stronger than polyester resin. Some epoxy resins form an amine blush when cured, which is easily removed with plain water. Some epoxy resins form no amine blush. Once the fiberglass layup of a boat is fully cured, neither epoxy nor polyester resin will form a chemical bond. Gelcoat is a polyester resin The only possible bond in a repair is a mechanical bond to a roughened surface. . Also some polyester resins contain a wax which forms on the surface and some do not. There is no reason not to use epoxy in a repair because it will be stronger than anything else you can use. The gelcoat finish will be a mechanical bond no matter what you use so it would seem to me that epoxy would be the material of choice. You'll get arguments and different opinions on this site so why don't you check it out on the internet. Google "gelcoat repair" and look up West System epoxy to see what they have to say about it.