To clean the bottom of a fiberglass boat, wash it as soon as boat is removed from the water. If residue is allowed to dry on hull, it becomes very difficult to remove. Most marine stores have cleaning compounds to help remove this scum, however effective chemicals can vary from harbor to harbor.
Tools & Technique
Hose the hull to loosen any surface growths, using a metal scraper remove barnacles – a palm sander is effective on difficult areas. Be sure to hose off hull as you work to avoid breathing in harmful dust. Areas where the gel coat or paint have been damaged in must be repainted, or repaired and waxed.