You will find some very good products for this job in motorcycle stores. They are intended to remove light scratches in Lexan, which is the material used in windshields. I have had good experiences with Meguiars' #17 and #18, clear plastic cleaner and cleaner/polish. Otherwise, another product that I can recommend is the Slipstreamer motorcycle windshield heavy-duty scratch remover. Obviously, if you can feel the scratch with your fingernail then it's not coming out for anything, but these are good.