Epoxy resins contain epoxides—remarkably reactive groups of molecules—that harden (or overcome) by means of chemical reactions, which might be induced either by combining it with other substances or heating it to some large temperature. Regular gelcoat formulated for use with polyester resins and vinylester resins doesn't adhere to epoxy surfaces, https://www.propanraya.com/id/produk/epoxy-fast-dry