Here is a write up about it. I have never tried it and I am wondering if anyone here has. "If you haven't heard of using a clay bar on your glass, you aren't alone; I hadn't heard of it either until last year, when VW/Audi were giving away free "windshield maintenance" kits that consisted of a 3M Perfect-It III clay bar and some spray. This is what turned me onto using it. If you doubt the effectiveness of using a clay bar, do this simple test: wash your car; dry it; use the spray glass cleaner of your choice on the windshield; and when you think the glass is as clean as you can get it, use a clay bar on it. It'll come up black with stuff it pulls off the glass. I know mine did. The clay bar manufacturers want to sell you detailing spray to use as the lubricant for the bar, but this is an especially bad idea for windshields, IMO. Aside from the fact that a car wash solution is a much, much cheaper way to go, it seems to me counterintuitive to use a spray that deposits stuff on your glass when you're trying to get the glass perfectly clean. I'm not a fan of Rain-X or any other "coating" product. In my experience they wear off quickly and make things worse than nothing if you don't maintain them. You'd be amazed how well really clean glass alone will shed water. I'm convinced that this is all that needs to be done, together with regular replacement of wiper blades. Your car is only 3 weeks old. I wouldn't expect the windshield to need much in the way of "deep cleaning", but I guess it depends on where you live and where it sits when parked. Eagle 20/20 (commonly available at auto supply stores) uses isopropyl alcohol instead of ammonia (which is what most household glass cleaners use). Works great, getting the glass very clean, virtually no streaking. Others have also strongly recommended Eimann Fabrik glass cleaner. Whatever you use, try using a microfiber cloth with it. Much better than paper towel or a regular buffing cloth for getting the glass clean and free of streaks. "