In the past, I've used regular Scotch Tape cut to the path of the wiper and fixed to the WS. Then just lightly dry-brushing the WS with a light earth color. Once happy with the results, remove the tape.
Maybe I'm lazy (or crazy), but breaking out the airbrush just to do a windshield, doesn't make sense to me. (I wouldn't do it.) I'd also be afraid of 'over shooting'. Light dry-brushing is IMHO, the way to go. (Do see my signature line.)
I know that when I was a driver and out in the field, I was always keeping the windshield (and lights if not tactical) as clean as possible... even if the only means of cleaning them, was by using my crusty old snot rag. Gave me a excuse to get out of the vehicle and stretch a little.