To clean a fiberglass bathtub, wipe away any dust with a soft cloth, spray the surface with a vinegar solution and clean tough stains with baking soda, advises DoItYourself. Use only nonabrasive rags and cleaners to avoid scratching the fiberglass surface.
Before cleaning the surface with vinegar and baking soda, wipe down the fiberglass bathtub with a damp, nonabrasive rag, DoItYourself suggests. After removing as much dirt as possible, make a vinegar solution by mixing white wine vinegar with warm water. Spray the solution onto the fiberglass surface, and allow it to sit for several minutes. Wipe it away with a soft cloth, using minimum pressure to avoid damaging the fiberglass. Rinse the vinegar from the bathtub with clean water.
Remove tough grime with baking soda, advises DoItYourself. Sprinkle the baking soda over the bathtub, or coat the bathtub with a baking soda and water paste. Wait a few minutes before spraying the sprinkled baking soda with the vinegar solution. This should cause the mixture to froth, a process that eliminates stains. Allow the mixture to sit and soak into the dirt for 15 minutes. After the mixture has lifted away the stains, rinse the fiberglass bathtub with warm water to remove all of the baking soda and vinegar.