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.
I think painting over or using comet mixed with baking soda would be the best. Honestly, it just might take a ton of elbow grease and labor. You can do it in sections to make it easier, if you go with painting it-be sure to talk to a specialist at home depot or something to see what kind would be the best. Good LUck.
The Works toilet bowl cleaner worked on both the toilet and the 50 year old tub in this house. They were nasty with rust and lime. I squirted some of The Works directly on the stains, let it sit and bubble a few minutes then rinsed. It took about 4 applications to get the tub a bright, shiny white. Looks almost new.