White vinegar can remove calcium buildup from hard water stains on fiberglass. It is acidic enough to get the job done, yet not too harsh on the fiberglass itself.
Simply drench paper towels in white vinegar and stick them to the affected area, or thoroughly spray white vinegar directly to the stains. Allow to sit for one to two hours, then use a sponge to rub the stains away.
Cream of Tartar
Make a paste of cream of tartar and hydrogen peroxide or cream of tartar and white vinegar. Make it a thick consistency and spread over the stains.
Allow to dry completely, then buff off with a clean rag.
A paste of borax powder and lemon juice will remove rust stains. Simply apply and scrub with a sponge.
Do not scrub too hard, as that can dull the fiberglass. Rinse well.
If there is still a hint of rust, repeat the process.
Calcium Rust Remover
A commercial calcium lime rust remover (CLR) is available at home improvement stores, hardware stores and in the cleaning product aisle at most grocery stores. This multipurpose cleaning solution removes calcium deposits due to hard water, hard water stains and rust.
In fiberglass shower enclosures, the rust that forms is likely due to metal fixtures within the shower enclosure. Wear rubber gloves and ventilate the room before using this type of product.