I think there is some kind of coating on the screen, probably a scratch-proof coating. A few years ago, on my first GS, whilst changing the screen, I felt it slip from my hand whilst about to lay it down on a cloth. It rolled on to a concrete floor which left it with a few light scratches. I tried polishing these out using something like T-Cut, then finishing with Duraglit for a finer finish. As I got past what I think was the coating the plastic got slightly softer and easier to work. Unfortunately, although the scratches were gone you could see a difference in colouration between the 'coated' areas and those that had been polished. It looked better than it was, but it was never going to be perfect again.
Bob