I finally got around to refurbishing my headlamps on my 20 year old ML320 W163. I figured if I could polish wood nicely then plastic headlamps would be no sweat. I started dry with 120 grit cloth backed abrasive to grind down the peeling and yellowing old plastic surface, then used it wet, going horizontally and then vertically until the horizontal scratches had gone. I repeated this with 180, 240, 320, 400, 600, 800, and 1200 wet and dry paper, cleaning it thoroughly between each grade with a hand held garden spray. I then moved onto 3M Micromesh, a rubberised cloth abrasive used for polishing space shuttle and Concorde wind screens. It is expensive but lasts for years. I used my oldest set which is nearly 30 years old and still going strong. I started with 1800 mesh, wet, then 2400 and 3200. Then using it dry I used 4800, 8000 and 12,000 mesh. Okay, there were some earlier scratches remaining and if it had been a nice piece of Rosewood, I would have gone back to 400 grit and started again, but I couldn't be bothered! I then used T-Cut to polish it a bit more. All the yellowing went and it is twenty times better than it was, although not quite as good as brand new headlights, but they are £500 each, so I am not complaining! The headlights looked worse at first than in the photos. The whole process took just under 2 1/2 hours.

I stopped at 3000 then used a polisher and some cut polish then finish wax and they turned out crystal clear. It is amazing how much change there is isn't there?
Only thing is doing it once a year. I tried to coat with clear once but wasn't happy with the result so just have done the repolish.