We have two formulas that work well on fiberglass tub and shower pans. The first is turpentine mixed with salt and the other is automatic dishwasher detergent. Here's how they work. Mix 2 tablespoons of turpentine into 1/4 cup of regular table salt. Remember we said turpentine, not paint thinner. Make sure to have plenty of ventilation and wear rubber gloves. Pour the mixture onto the surface and scrub vigorously with a nylon bristle brush. Wipe up the excess cleaner with a paper towel and let it air dry before you throw it in the trash. If that doesn't prove to be strong enough, try automatic dishwasher detergent.

First, plug the tub or shower drain and fill with water to about one inch. Sprinkle an even layer of the detergent over the water and watch it sink to the bottom. Let it stand overnight. Next day, drain the fixture and use the paste that's left to scrub away dirt and stains. Repeat the procedure if necessary.