I have seen various articles and used many products in trying to remove stains. The best product I have found that works particularly well on rust and mildew stains is a product called "FSR Fiberglass Stain Remover" manufactured to remove stains from fiberglass gel coat. Just apply, wait a few minutes and rinse it off with water. Most all major marine catalogs and retail stores carry it and only costs about $8.00 - $10.00.