Remove soap scum with baking soda and water. Once soap scum accumulates, it can be difficult to get off with regular bathroom cleaners. Mix water and baking soda until it has a paste-like consistency and apply the paste to the soap scum areas. Leave the applied mixture alone for a couple of hours and then rinse it off with warm water.
Use an acid to dissolve stubborn soap scum. More challenging soap scum might need an acid, such as vinegar, to help dissolve it. For best results, soak a cloth with vinegar and cover the stained area with it for about an hour.
After removing the cloth, clean the area with warm soapy water and a soft sponge. Stronger acids, such as diluted muriatic acid, should only be applied while wearing rubber gloves and as a last resort.
Bleach dark stains with hydrogen peroxide. Deeper, darker stains may be challenging to get rid of with regular cleaning products. Place several layers of clean white cloths over the stained area and pour hydrogen peroxide on top. Let the hydrogen peroxide soak in overnight and in the morning, rinse and dry the area.
Use acetone or paint thinners on extreme stains. Difficult stains can be removed with acetone, which is the main ingredient in nail polish remover, or paint thinners such as turpentine.
Soak a clean white cloth with the solution that you’re using and gently rub the stained area until it returns to its natural color. These solutions are fine to use occasionally on stained fiberglass, but should be used sparingly.