I mask it and use an air brush. Like Dan does I give it a nice thin coat a first then a couple of medium coats untill it's nice and dark. I think I've really perfected it on my RX-7 and S2000. Windshields should be painted on the inside
How you cut the mask and do not scratch the windshield?
The same when masking over painted surface. Knife leaves scratching marks on it