I tried this. I just used QD as a lubricant. It got off all the little sap bumps and such. However, the windshield still looks crappy. It's because it's scratched, though. The clay bar does work, and will do exactly what it does for paint.
I have used clay for glass before. It does work, and will really help smooth out wiper operation as well. Use either the QD lubricant supplied with the bar, or a capfull of car wash in about 12-16 oz's of water. :up
You will not really be scared of swirls/scratches while using clay on the windscreen, but still need to beware. I have tried using Rain-X Cleaning Compound...it won't achieve an ultra smooth finish as compared to using clay bars.
Yep, clay works great. I tried it yesterday on my glass and black painted plastic trim around the windows and followed up with a coat of Z2. They're now smoother than ever.
I have a TSB (Technical Service Bulletin) from Audi recommending that you clay your windshield at least yearly. Last year they even sent me a coupon for a free package of 3M Clay and Quick Detailer!:up