Sometime amateur car detailer here, and one of the things that works good on car windows, (not windshield) is very light car wax, not the carnauba, but the silicone types. It makes the glass super smooth, resists spots, water runs right off. I've used it on my patio doors and even a glass table, makes the glass like Teflon. Anyone brave enough to try it on the iPad screen? I know some one posted they used it on their screen protector, to make it slicker.