Safe cleaning techniques to remove soap scum from a fiberglass bathtub include using borax, baking soda and vinegar. Ammonia also safely removes soap scum. For problem areas, a wet pumice stone can be used to very carefully scrape away scum remnants.
When using borax, mix 1 cup of borax with 2 tablespoons of dishwashing detergent and 1 or 2 cups of water. Apply it with a soft bristle brush and allow to sit for 15 minutes, and then scrub until the soap scum disappears. Rinse the tub thoroughly with hot water. The process for using baking soda is exactly the same, aside from the substitution of baking soda for borax.
To use vinegar to remove soap scum from a fiberglass bathtub, mix 4 cups of white vinegar, 4 cups of water and a tablespoon of dishwashing detergent in a spray bottle. Spray the mixture on the soap scum, and allow to sit for 15 minutes. Scrub away the scum with a soft bristle brush, and rinse with hot water.
The use of ammonia requires proper ventilation. Open a window, and then mix 1 cup of ammonia and 2 cups of water in a spray bottle. Spray the solution on the soap scum, and allow to sit for 30 minutes. Scrub any soap scum with a soft bristle brush, and rinse away the remnants with water.
Pumice stones should only be used in the event of soap scum that cleaner will not remove. Make sure to wet both the stone and the surface to be cleaned, and gently rub the pumice stone across it until the scum disappears. Rinse away residue with hot water.