Fiberglass can easily become discolored over time due to oxidation, fading from the sun or other environmental conditions.
To remove discoloration from fiberglass, you’ll need to use a mild abrasive cleaner. Wet the surface of the fiberglass with water and apply a small amount of an approved cleaning product such as Soft Scrub. Gently scrub the discolored area in a circular motion with a sponge or soft-bristled brush to remove the stain. Rinse off the cleaning product and inspect for any remaining stains.
If there are still spots you can use a stronger cleaner such as Simple Green Pro HD Heavy Duty Cleaner, which is specially formulated for fiberglass. Be sure to follow the instructions on the label and take care not to use too much cleaner.