Hello bear031 and Welcome Back!
I have a few suggestions to try but if the porcelain is damaged or has lost its finish, be careful, because most cleansers can actually make it worse. If this is the case, you may have a permanent stain and you should consult a professional.
If the porcelain is still in good shape, you can try these tricks: Baking soda works wonders on many stains. Just wet the tub, sprinkle some baking soda and a few drops of dish soap, scrub well, and rinse thoroughly.
Mix cream of tartar with hydrogen peroxide to form a paste. Coat the stained area and leave on until the paste dries. Then scrub the residue off with a non-scratching scrub sponge. The stain should come off with it.
You can also try soaking the stains in a mixture of lemon juice and salt, then rinse clean.
If the stain is still there try Tang Breakfast Drink ™ or lime Kool Aide™ to ‘eat’ through the lime or rust deposits. Citrus counteracts the chemicals sticking the lime deposits to the porcelain. The acid in the mixes oxidizes the rust.
Here are more tips to
remove hard water stains.
Good Luck and let me know how it turns out!
Mrs. Clean