Put on rubber gloves and fill a bucket with hot water.
Read the manufacturer's instructions on the label of the cleaner and add the recommended amount of mild tub cleaner to the water.
Use a sponge to work up some suds in the bucket and firmly rub over the fiberglass surface. Use small circles to be sure you cover the area thoroughly.
Rinse the surface with a clean bucket of plain water. Rinse thoroughly to be sure all of the residue is completely gone.
Allow the piece to air dry. Buff it gently with a clean dry towel to remove any water spots.