The scale is usually a deposit of hard water minerals. Usually CLR or the Works will remove this given enough time. Check out the site below about "The Works". If the stains have some rust colour in them use Navel jelly to remove these. Plain mineral deposits that are very heavy can be removed with muriatic acid (hydrochloric acid) found at building supply and hardware stores. Use Gloves with this. A nylon scrub pad should not harm fiberglass, but if the fiberglass is already damaged, use the resin from an auto body repair kit to finish the surface. Waxes will just come off as the tub is used.