I think it is called jewellers rouge, might have the name wrong, but it is for very finely polishing out scratches.
Or do as I did:
I tried a kit a while back from e-bay and it did work, I noticed no distortion of the screen as it was worked (electric drill and pad) over an area of fine scratches caused by an ice scraper. If it was deeper, such as a wiper arm scratch, then I am sure the optics would change.
I didn't want a new screen as I had read about problems with auto lights/auto wiper by a new screen.