Epoxy resins include epoxides—hugely reactive groups of molecules—that harden (or cure) by chemical reactions, which happen to be caused possibly by combining it with other substances or heating it to the large temperature. Typical gelcoat formulated to be used with polyester resins and vinylester resins doesn't adhere to epoxy surfaces, https://www.propanraya.com/id/produk/epoxy-fast-dry