I've never had a chance to test on plastic, i.e. windscreens, but as a long time commuter from LA to the central coast via the central valley of Calif, (bug haven) but I always used Bon Ami cleanser on the windshield and chrome of the cage. Saw that on "Shade Tree Mechanic" ages ago. As the product says, "It hasn't scratched yet" and it didn't. Great bug remover.