How do you get tough stains out of a fiberglass tub?

You need to handle stains in fiberglass tubs and showers carefully so that you don’t damage the delicate finish. Try letting the stain soak in castile soap for up to an hour, then rub the area with a soft cloth or sponge in a circular motion. Rinse the area thoroughly when you’re done. If that doesn’t work, you might be able to scrub the stain out with a baking soda paste and a soft cloth. For ultra-stubborn stains, acetone-based nail polish remover or turpentine may work. Let the solvent sit on the stain for up to an hour, then scrub the area with a soft nylon brush. Wipe it clean with a damp cloth or sponge.