Match the grit to the stage. Knock down cured laminate or heavy gelcoat with 80 to 120 grit, fair and smooth with 220 to 320, then wet-sand gelcoat for polishing with 400, 600, and on up to 1000 or finer before you buff. Sanding sponges follow curves better than flat paper. Always sand cured surfaces and wear a dust mask, since fiberglass dust irritates skin and lungs.