S.K.
There's a product called Bath Brite. It gets rid of rust on fiberglass showers. I've used it many times for other things too. I'd put it in full strength and let it sit for about 15 minutes and see if it gets it out. You can buy it at Walts food, or at MTS Supply in Schererville, ###-###-####, or I know Leep's supply carries it also. It's pink or clear and smells like wintergreen.