Orange juice. Sounds crazy but if you get the cheapest OJ the kind not in the frige section. Put it in a spray bottle and spay it on a let it sit it then rinse off. . I had a problem with hard water and then one day there was rust in the water that turned the tub orange. The water company as going to replace the tub. I don't know why I decided to dump an expired bottle of OJ in the tub but I did and it cleaned the entire thing no hard water build up and no rust. The tub never had to be replaced.