Worst Solutions For The Long Run: Dishwashing Soap - Liquid dishwashing soap might wash off visible dirt and grime. But it also opens pores in the fiberglass, allowing it to hold dirt and trap light instead of reflecting it. Ammonia-based Cleaners - Window cleaners and household spray cleaners have ammonia, which dissolves acrylic in vinyl, fiberglass and plexiglass. Soon after using it, crazing, hazing and cracking will destroy the look of your boat. Traditional Paste Wax - Aside from an unnecessary workout, many traditional waxes lack protectants to shield oxidation. Bath Towels - A towel can hold only so much water before it starts to just spread the water around. Use the towels to wipe up spills, and leave the windshield and fiberglass wipe downs to something more absorbent. Bristled Brush - It might get the toughest scum lines out, but a hard-bristle brush can leave tiny scratches and take away the protective layer from the gelcoat. Think of it like a toothbrush on sensitive teeth.