quote:

from: How to Clean Anything.com
Cleaning Spotted Glass  
Some parts of the country are burdened with extremely hard water. Hard water is water that contains excessive trace elements including calcium, iron, lime, and other minerals found in streams and rivers. These trace elements remain on your glass when water from sprinklers or washing air dries. These hard water spots attach themselves to glass like white on rice. If left on your glass for very long, hard water spots will not wash off with shampooing or regular glass cleaners.

In my experience, there are only two ways to remove hard water spots. The first is to use distilled white vinegar. If your water spots are mild, you can dilute the vinegar 2:1 with water. Heavy spots may require pure vinegar. Either way, the vinegar will not harm you or your car. Vinegar is a very mild acid, which dissolves the alkaline-based mineral water deposits with ease.

If hard water spots are allowed to remain for more than a week or two, they may etch the glass. So, even though you have removed the minerals causing the water spots, the spots remain as damage to your glass. In these cases, it is necessary to polish the glass. Until recently, there were a number of glass polishes on the market. The two best products were Eagle One Glass Polish & RainX Glass Polish. However, both of these products have been discontinued due to little demand.

In my experience, any light (low abrasive) car polish that does not contain paint feeding oils will work as a good glass polish. Eagle One Scratch Remover is a good example of a polish that does not contain oils and works well on glass, as does P21S Multi-Surface Finish Restorer. There are two ways to use these polishes. The most gentle is to use an old t-shirt to polish the spotted area and buff to full luster. The second is to use the polish with #00 or finer synthetic steel wool to remove the spots then buff with an old t-shirt.