I used a face shield when grinding on welds and a lot of times the fine dirt and grindings were attracted to the shield from built up static. I would use anti static spray on my shield inside and out and it seemed to make a shield last a lot longer. I think that most of the scratches come from stuff sticking to the shield and wiping it off, if it don't stick then you don't have to wipe it off as often.
I get this stuff from Wal-mart, they have small travel size cans for a buck and it goes a long way, spray it on and wipe it off with a soft rag, works on powder hoppers and dippers also.
http://cn1.kaboodle.com/hi/img/b/0/0...=1301717089000