I've been painting (part-time) for a while, and agree with spraytech. Fiberglass is rigid enough, unless it's very thin, to resist flexing. I've hand-built fiberglass bodies for some of my projects and have never added any flex agent to the paint. I used acrylic urethane paint with the appropriate primers and such, and the paint has been on one of them for at least ten years and still looks great. And gel-coat is one of the most brittle surfaces out there, as far as I'm concerned. It's great for its intended use, but should be used in moderation. Just my $.02