Fiberglass laundry tubs are extremely popular because they are durable and not likely to chip. But many household cleaning agents can damage a fiberglass tub—especially stain removal products. In order to remove stains from fiberglass laundry tubs, you will need to use delicate and very specific cleaning supplies.
Put on rubber gloves and apply a spray cleaner to the tub surface before wiping down the entire tub. This will remove any run-of-the-mill dirt and grime, making it easier to target problem areas.
Pour 1/4 cup of borax into a bowl and combine it with lemon juice until you have a thick, spreadable paste. Generally, this will take about a quarter cup of juice, but add it slowly so that the liquid does not become runny. Apply the paste over rust stains and leave it in place for about 10 minutes. Wipe off the paste with a damp rag. If the stains lighten but remain visible, reapply the paste and scrub it off with a plastic scrubber, which will not damage the fiberglass.
Make a paste of vinegar and baking soda. Combine 1/4 cup of baking soda in a bowl, then add vinegar in small amounts (about a tablespoon at a time) to create a thin and spreadable paste (typically, this will amount to about 1/3 cup of vinegar). The mixture will fizz as you add the liquid.
Apply the paste with a rag to the inside of the tub. Let the paste sit for about 10 minutes, but do not let it dry out. Scrub the tub with the rag and the plastic scrubber until the paste has removed any remaining stains.
Reapply the spray cleaner to the tub and wipe the surface clean to remove any remaining residue. Rinse the tub thoroughly with water.