MarkDW
I have had some luck sanding windshields out with successive finer grits and then using Novus plastic polishes
As mark suggests, if you can sand the mark out (assuming not too deep to sand out). then use a polishing kit of increasing grades until you restore the clarity of the part.
start at 2500, 3000, 4000, 6000, 8000, 12000, then Novus 3, 2, 1. don't skip any step and sand always in the same direction.