I use Oxolic Acid dilluted quite a bit. It is Wood Bleach at Home Depot or some such store. If you use gloves and a sponge all you do is put it on, wait for it to work then wipe it off. If you dilute it right it will take about 5 minutes for the fiberglass to come up to almost new. Rinse it off then use a teflon finish to coat it.

Later,
Dan