media blasting fiberglass is quite touch.y Becuase the blasting pressure changes all the time when your hand gets closer to the surface. Therefore this would cause up and down spots and you would eat through the gelcoat surface under the paint. Sanding with the proper grit and a good couple of coats of primer would be better. apply primer once you get to the gelcoat surface.
media blasting metals is fine, because you can rarely eat into metals unless it is a soft aluminum.
Be careful is all Im saying.