The best way to clean a fiberglass tub, or any other form of fiberglass, is to use a non-abrasive cleaning product, such as a general all-purpose household cleaner or baking soda mixed with water. Simply apply the chosen cleaning liquid with a sponge. Avoid using abrasive scourers.
Some particularly tough stains may be difficult to remove using the above method, however. Oil-based paint or liquid, tar, and iodine, for example, can be particularly hard to remove, and can cause stains. In order to remove stains from fiberglass, applying turpentine or acetone to a non-abrasive cloth will often remove the stain completely.
Areas that are stained should be rubbed gently, taking care not to apply too much force to the cloth or sponge. Additionally, due to the highly flammable nature of both turpentine and acetone, the cloth, fiberglass and containers of the liquids should be kept well clear of excessive heat or flame.
A fiberglass tub can also be kept in good condition, and require cleaning less often, by polishingit with white automotive wax. Additionally, making sure the tub or shower is dried after use will prolong the life and quality of the fiberglass itself. Giving the tub a quick wipe to dry it after use will ultimately save on cleaning further down the line.