[header] How to sand fiberglass [title] Allow the fiberglass to cure in the sun. [step] If you are working with a new fiberglass component, it will have a thin layer of gelcoat on its surface. Gelcoat is an epoxy or resin-based compound that is used to line the molds when making fiberglass components. 
Before sanding, let the fiberglass sit out in the sun for 2 to 7 days to cure the gelcoat. This process releases any air pockets from the gelcoat that would cause problems when sanding and painting.