Here's a write-up I did on restoring plastic headlights on maxima.org:
http://forums.maxima.org/showthread.php?t=375480
It can be time consuming, but the results are well worth it. I have since done it again to my headlights to improve some trouble spots, but this time I removed them from the car. This made it much easier. I mounted the lights in my portable work-bench and did them in my living room while watching TV. I also used the Novus plastic polishes because I've had great results with them on different clear plastics. With using a PC, I couldn't tell a difference polish wise between the Novus and Meguiar's products I originally used in the write-up. If I was polishing by hand, my experience tells me the Novus would have yielded slightly better (or atleast faster) results. But since I have a PC, I used it.
If your headlights are only yellowed, Mother's Mag and Aluminum Polish works wonders for removing it. But most headlights that are yellowed will also have small pits and scratches, so wet-sanding would yield better results. But the Mother's is nice for touchups or when time is short. Use it just like you would on metal, only with a softer touch.
In the end, product selection is not as important as technique and patience. Just about any fine abrasive polish could work, even those made for paint (which is what I used in the write-up). For sanding paper, I recommend starting at 1500 grit if lenses are bad; 2000 grit if only slightly pitted/yellowed. If I could find 3000 grit local, I would add that step in order to cut down on polishing time.
For protection after sanding/polishing, I have been using Meguiar's NXT and 4 Star Ultimate Paint Protection, both with good results. Any good sealant should protect decently. I think a carnauba based wax wouldn't last as long due to heat. My headlights get "waxed" everytime I do my car. I think 303 Protectant would also be good to use because of its phenomenol UV protection, but it would need re-applied every 5 weeks or so to maintain UV protection according to 303's information. Meguiar's NXT and most other sealants should last much longer than that. My car gets NXT atleast every 2 months whether it needs it or not, so that's about the longest my headlights have gone between waxings. It easily lasts that long. Plus they add a nice "gloss" to the lens, and aid in future cleanings.
Anyways, hope this helps someone. If you have any questions, PM or email me and I'll be glad to help out if I can.
Dave
