If the shield is really scratched up and clouded looking, the hard "scratch coat" is probably starting to peel off from repeated clean offs with a glove or towel. I have had success with plastic polishes, but you pretty much have to get all the peeling scratch coat off to get it clear again. Did it to a Modular Ski Doo shield, they cost way too much to replace. It worked well, but then you have to be carefull because the hard coating is gone. Something to consider...
