[C-Man]

There's no problem using clay on the windshield. I usually use new clay for the paint, and then after it is used a few times, I'll use it on the glass. The reasoning is that the paint can get scratched by any crap that the clay picks up, but the glass won't. Also, cut the clay up into smaller pieces, that way if you drop it in the dirt, throw it away and use a fresh piece. Do not use dirty clay on the paint.