I have used the same method tommudd describes. . Used to work at a couple dealerships while in college and sometimes new cars would have nasty crap all over the windshield and we would also have to get some over spray off windows from the body shop occasionally. We used Sprayway window cleaner ( I still use it on all my cars, works great imo) and would cover the windshield and then use a piece of super fine steel wool and lightly run it over the windows and it would remove any impurities off the glass. Wash it off and then clean the windshield again with glass cleaner and use a final inspection spray ( final detail liquid) and it was crystal clear.