So I have had these crazy water stains on my car window, I think it came from the neighbors sprinklers and we have very hard water here. Seriously nothing would clean it off and I literally tried every thing I could research, all kinds of acids, Bartenders Friend, Aluminum Brightener, etc. When I was at Ace hardware looking to modify a scrub brush to a drill head (yes I was that serious) the sales clerk mentioned I should try this stuff called Diamond Magic. It was expensive, but since I had already spent a half day scrubbing I was winning to try anything.
I kid you not this stuff is FREAKING AMAZING. You still have to scrub (a little bit) but nothing more than it would take to clean your tub. It also works on all kinds of other surfaces. Truly and incredible product!