Depending on the extent or depth nothing might do but a new windscreen. However, I ran my Porter Cable over mine with my current favorite polish product:
BLACKFIRE Scratch Resistant Clear Finishing Polish, final polish, clear coat polish, car polish
As long as you are careful with pressure and the seams where the water barrier begins it wont hurt a thing. IMO, if anything is going to work it will be the PC, you simply cannot duplicate its abilites by hand.