My dad is a professional detailer and for water spots as you describe my suggestion would be acid.

I don't know where he buys it from (it's from a supplier) but I'll ask him tomorrow. What you would do is spray acid on a wet car (in the shade and do this in sections) and then rinse after about 15 to 20 seconds, you don't want the acid to stain the paint.. AND BE VERY CAREFUL WITH IT ON WINDOWS! Only do windows is the glass if cold and in the shade... You don't want to etch the glass. Acid is some crazy stuff, but I think it will do wonders for you and definitely get these spots off..

Again I'll ask him where he gets the acid from. Also as a quickie try steel wool on the windows (fine steel wool) it might take the spots off and if not make it easier for the acid.

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76 MG Midget
78 Datsun 280Z
86 Peugeot 505
01 Subaru Outback

“Owning a car is like owning a dog. Every now and then, it craps on the carpet — but it’s still your dog.” - Jeremy Clarkson