Fiberglass

Fiberglass is a resin with embedded glass fibers that is used in many applications from skylights and shower doors to bathtubs, sinks, canoes, and other common objects. As long as it’s not scratched, fiberglass is easily cleaned. The more scratches that penetrate the surface of the finished layer, the more dirt gets trapped in the scratches. These tiny scratches can also be unsightly and lead to performance issues.

Avoid dry-scrubbing fiberglass and use only soft sponges or cloths. Do not use abrasive pads and cleansers when cleaning fiberglass. Use only mild cleaners; you may even be able to wipe it clean with plain water. Avoid strong solvents, such as paint thinner, acetone or high alkaline cleaners, which can damage the fiberglass’ plastic resin. If you are unsure of your cloth’s or mild cleaner’s effect on fiberglass, perform test cleaning on a small, corner piece of fiberglass before proceeding.  

Apply car wax to fiberglass occasionally after cleaning just as you would to your car’s surface — dirt just slides off the protective coating. However, use a bath mat for safety’s sake if you wax the bottom of a fiberglass tub or, better yet, don't wax it at all.

Reference:

 

Don Aslett in The Cleaning Encyclopedia: Your A to Z Illustrated Guide to Cleaning Like the Pros.