MIne had pitting and score marks all over it when I bought it. The score marks were in line with the travel of the wipers so some nitwit had obviously not been changing the wipers as frequently as they should and ended up just grinding contaminant into the screen.
I used wire wool to 'polish' the glass and it helped but didn't exactly cure the issue. I only really ever notice it in low, bright sunlight however, which doesn't seem to occur so often.
Short of a new screen I doubt there's much else you can do. Would be nice if there was some form of filler for micro-pits like the stuff you can get for chips.