not sure since I never used wax in the gelcoat mix, but that could possibly be it..

I've sprayed thinned gelcoat with the Prevail for smaller jobs with good results.

Thinned it to just a little thicker than car-paint consistency and sprayed several layers to build thickness (letting it flash off between coats), and then spraying that green stuff that smelled like chloraseptic to seal the top coat (mold release)

Course, I'm in the tropics (high heat/humidity) so flashing off took about a minute..