Muriatic acid and safety gear plus a little elbow grease will knock those stains back to white. If the thought of using acid scares you (it shouldn't; just be smart about it) you can use a toilet duck type cleanser. These have a diluted form of the same thing in there make up and that's what keeps your toilet white. I personally have reclaimed a fiberglas tub enclosure from hard water stains as orange as a pumpkin to showroom white with nothing more than a cloth, acid, and a respirator. Total time was maybe 10 minutes.