I forget who told me about the stuff, it may have been here. FSR, Fiberglass Stain Remover. It comes in a little tub, looks like the stuff inside those freeze packs, same blue color anyhow. It works great! It took several applications but the stains just vanished. We had rust or pollution stains that came out of our cockpit scuppers and went down to the waterline, man they were ugly.