I've used lacquer thinner with good results. I've heard acetone works too, and in a pinch, wax and grease remover with a little elbow grease. Just gotta rub a little harder. However, lacquer thinner and acetone will remove paint, but since your having it painted I don't think it will matter. If you get any on your new paint when you install the glass, I'd stick with wax and grease remover.