Bruce--

I spent a couple of days last week repairing fiberglass kayaks. We were able to get our small gelcoat patches to harden in the open air. I'm not the chemist, but I think we were using resin that contained wax. After it was hard we wet sanded and buffed the surfaces. We got excellent results.

Our problem was one old boat that had enough repair spots to make patching seem like a poor choice next to completely sanding the surface, repairing the gouges and, we hoped, spraying on all new gelcoat. We couldn't find a balance between thinning enough to get rid of orange peel and thinning too much and getting runs.

I was hoping to find someone who had successfully sprayed gelcoat on the outside of a hull (not into a mold). But as Sam and Hans said, it seems more common to spray and sand smooth or use a 2-part paint for the final finish.