Cam
No prep for underside of glass, just air cleaned it Since underside is rough anyway, don’t expect a smooth finish, it is thin enough to conform to glass but not thick like some bed liners. It was cleaner to brush it on because I didn’t want this stuff everywhere....it won’t come off. I only needed one coat...it tacks up quickly so I mixed half bottle at a time. If you do the last brush strokes in one dirction after it’s applied it tends to smooth over very nicely and you really don’t see brush marks. Makes the bonnet underside very finished looking. I went ahead and did the underside of ALL body parts even if it won’t be seen. One box (4 bottles) did my entire car then some.