Most of my experience is with Southern Polyurethanes (SPI) epoxy, whose properties may not translate exactly across the board to other epoxies.. With SPI epoxy it is waterproof after three coats. One doesn't normally spray that many coats on a windshield opening, and to resist any risk of chalking from UV exposure, I'd say your best bet is to media blast any pitted/rusty areas and address repairs as needed, epoxy prime, and then topcoat. (observing all mfr recommended flash/recoat times)
