http://men.msn.com/articlepm.aspx?cp-documentid=760921A Basic Problem
Q: I have a sprinkler system in my front yard that splashes water onto my car on breezy days. I think the water droplets that evaporate from the side-window glass leave mineral deposits behind. Oddly enough, the paint doesn't seem to collect any deposits, nor do the windscreen or the back glass. I've tried every window cleaner I can imagine, but nothing gets rid of these annoying stains. My son-in-law suggested I try using hydrochloric acid to dissolve them, but that really doesn't sound like a good idea to me. How can I remove these spots?
A: Bad news: They probably aren't mineral deposits. The glass has been etched by corrosion. Here's the phenomenon: Your municipal water--or whatever water supply feeds the sprinkler system--may have a high pH (it's alkaline, or a base), or you may have had some highly alkaline surface dust on your glass, which dissolved into the water. This high-pH water then slowly dried in the sun. As the volume of water decreased, the concentration of minerals, and consequently the corrosive quality of the solution, increased. This could have etched the surface of the glass, leaving not a deposit, but a ring that is below the surface of the surrounding glass.
Worse news: You can't remove these rings with glass cleaners or acid. You have three options:
1. Get an auto glass shop to polish the surface of the glass.
2. Polish it yourself. This requires a felt wheel, a variable-speed electric drill and a kit of various grades of cerium oxide abrasive. Think of grinding and polishing a telescope mirror.
3. Replace the glass.
Why didn't the water also etch the windshield and backlite? They use a more durable, laminated glass. Most important, how do you prevent this from happening again? Keep the windows clean and waxed so the water doesn't pool on the surface in big spots.
And, as an aside, the only acid I would suggest you try is straight household white vinegar. Put some on a rag and don't let it dribble. Don't rinse until it has dried, to avoid getting any acid into the door.