Stains are common on almost any surface, but they can be quite frequent on fiberglass material. Common fiberglass stains may be caused by bacteria with a high iron content, or water filled with brass and copper. You can use a number of different ingredients found around the house to treat the stains, such as lemons, known for their acidity.
Wash the entire fiberglass surface with a mixture of dish washing soap and water using a soft sponge. With fiberglass, it is very important not to scratch the surface because scratches are highly visible and difficult to remove.
Put on gloves, then mix up a paste, by hand, using borax and lemon juice. Use just enough lemon juice to thicken the paste so it can be easily spread over the stain without dripping. Lemon juice is completely acidic, and acid eats through rust. Let the paste sit on the stain until it hardens slightly then rinse it off with warm water.
Mix up another paste using 1/2 cup of scouring powder, 1 cup of peroxide and 1/2 cup of cream of tartar. Cream of tartar is a 50/50 combination of mild acid and water. It makes a paste that is more acidic than lemon juice. Combine the cream of tartar mixture with a small amount of scouring powder and peroxide. Apply the paste over the stain with a spoon and let it stand for half an hour. Do not mix or spread this mixture with your hands because a pure, yet mild, acid is being used. Do not scrub the fiberglass as it will scratch easily.