If the scratches are "noticeable" without the sun shining through the shield, IMO, its about time for a replacement!
F4 shields do no scratch as long as cleaned as instructed!
I got a farily bad scratch on lexan w. shield I had to sand it then polish... I started with 600 until scratch was almost gone then then worked my way to 2000 grit then used 3-m 3000 wet disc from 3-m headlight polishing kit.. then polished with compound that came with kit..scratch is gone I don't notice any distortion but my eyes are as old as me!!