Wet Sanding!
Clean it first is the key before you start. I have an old dishwasher that I throw the disassembled unit into before I start to get it throughly clean!!! If there are scratches on the unit, I wet sand the plastic VERY CAREFULLY! This takes alot of time, it usually takes me upwards of 14 days to wet sand a hood to my satisfaction, going down to 1500 grit, then 2000 grit. Go to 1500, or 2000 ONLY after you are sure all the scratches are out. At 2000 grit the plastic just about shines. I then use plastic polish compund on my buffing wheel and **CAREFULLY** polish the plastic to a high gloss shine. Remember to tape off any decals, and DO NOT SAND silk screen printing such as "low normal high shag" After this I use automotive wax such as Mothers. My machines never get scratched again because after they are restored they are not used again. I only run them once a month to keep the lubrication moving. I do not restore machines I use in this fashion. I did this for someone who became a good friend of mine, Michael Labue for the 63 he got from me. That 63 hood shined like a diamond when I was done. It was like nothing I've ever seen. For me restoration is a labor of love! It takes alot of practice so get an old hood to start on, one you don't care about damaging!