Try this. Mix some gelcoat and put enough to make a good size chip on a mold surface. After it cures, take it off, take a rag with acetone on it, and try wiping both sires. The exposed side will leave gelcoat on your rag, the mold side won't. That is because gelcoat is designed to make a chenical bond with the fiberglass; it isn't fully cured. Oh, and i've been a custom boat builder for the past 8 years, professionally. How about you?