You get the same problem on glass shower screens etc that get constantly exposed to water, apparently it's the minerals and stuff in the water that can actually attack the glass and leave it marked. It's not calcium build up as some people have suggested. It's probably less of a case on car windows as most of the water to hit them is rain water which doesn't have the same minerals and chemicals in it as tap water!
Fine steel wool or some sort of mild cutting compound would be the way as suggested.
Bookmarks