A technique that will save you a lot of sanding when you use fiberglass and epoxy is to take a strip of Saran Wrap (like used in the kitchen as a food wrap) and put it over the fiberglassed area, smooth it out with your hand so no wrinkles or folds are visible and leave in on overnight while the epoxy cures. When you take the wrap off the next day, the finish will be smooth as glass and not require any sanding.