Fiberglass needs to be stripped, sanded and primed before it is painted. Stripping generally consists of manually stripping away any waxy residue or finish from the fiberglass using a fine-grit sandpaper, such as 320-grit. An easier but less environmentally friendly option is to strip the fiberglass using chemicals. It is best to consult a professional before using chemicals to strip fiberglass to make sure the material is not damaged by the process.
Once the wax and finish have been removed, the surface of the fiberglass needs to be roughened so the primer and the top paint can effectively grip the surface. If the fiberglass is not sanded prior to the priming, defects can form in the paint, and it may flake or peel away from the fiberglass.
Use a fine-grit sandpaper, such as 80- or 120-grit, and an orbital sander. The fiberglass can also be sanded by hand; however, this is more tedious and less precise. Do not sand through the outer clear gel coat that encases the fibers within the fiberglass. Completely sanding through the gel coat can cause the fibers to press out of the fiberglass when the paint is applied and ruin it.
Once the fiberglass has been sanded, evenly apply a fiberglass primer to the surface of the material, and allow it to dry. Apply two additional coats to the fiberglass before applying the paint finish.