Fiberglass showers and other bath fixtures are available in a wide array of styles to fit anyone's décor. But hard water stains seem to be drawn to fiberglass shower as if it were a stain magnet. These stains have long been thought to be difficult to remove without damaging the fiberglass with harsh abrasives. However, you can effectively clean your shower to a sparkling clean state using harmless non-chemical products that you already have in your home.
Place a few washcloths into the bottom of a plastic pail, and dampen them with distilled white vinegar. Place the cloths on the stains in your fiberglass shower, and leave them in place for 15 to 20 minutes. Vinegar contains acetic acid, which effectively softens and loosens built-up hard water deposits that trap dirt and oils.
Remove the washcloths from the shower. Scour the hard water stains with a scrub brush or non-abrasive scrubby. A toothbrush works well in hard-to-reach spots. Rinse the shower thoroughly with hot water. Use clean towels to dry it completely to avoid spotting.
Repeat Steps 1 and 2, except substitute plain lemon juice for the vinegar, if any stains are still present.
Mix about half a cup of baking soda with an equal amount of plain uniodized salt if the stains were not completely removed by the vinegar and lemon juice. Stir hot water into the dry mixture, a little at a time, until it forms a stiff paste. Spread the paste to completely cover the stains in your shower, and leave it in place for 15 to 20 minutes. Then scour them using a scrub brush. The salt will act as a harmless abrasive agent while the baking soda will lighten the stain. Rinse your shower thoroughly with hot water. Baking soda is heavy and may not rinse away immediately, so you might need a second or third rinsing. Dry the tub completely with soft, dry towels.
Use car wax on your completely dry fiberglass shower to protect it from future hard water stains (follow the manufacturer's instructions).
Use dry soft cloths to wipe away the dried car wax. Buff your fiberglass shower to a beautiful shine with another clean, dry cloth. The wax will go a long way toward repelling hard water stain and dirt build-ups. Use it on your other bathroom fixtures and tile, too.