I have been asked to repost this with the pics....
I don't waste a lot of time on this job, to get them like brand new (seriously, not close but new) in a few min here is how i do it.
Mask off the light well with 3-4 layers of tape or remove it, Sand with an orbital sander and a foam 3000 grit foam disc. These come in 6 inch so use a old 5 inch disc and mark the pad then cut it down to use on the common 5 inch orbital sanders.
Dip the pad in a shallow pan of water so the pad holds water and stays wet, sand it till it looks perfect.
Polish it with compound and swirl remover after sanding using a buffer, i use a 6 inch rotary, the orbitals are painfully slow. Do not skip this step, it makes the difference between looking good and looking new!
Wipe 2X with alcohol, let dry and blow a couple of coats of clear on it. I use 2K when i paint something else but U-POL has a nice UV resistant rattle can clear. The picture is all reflections but it is 100% crystal clear.
You can 3000 sand the clear and buff it again but you don't have to.
This one was pretty hazed, 10 min per light total prep time.
You can buy the these product single at an auto body supply store. The disc are abut $3eahttp://www.amazon.com/Meguiars-S6F3000-Professional-Unigrit-Finishing/dp/B004QJIY36http://www.amazon.com/U-Pol-Products-079...s=upol+uv+clear
I use a Dewalt variable speed orbital and run it on about 2-3 for dull hazing, even crusty yellow will come out flawless just increase the speed a little and keep the pad wet.
1 disc will easily do a pair of nasty light so for a $3 disc and a $20 can of UV clear (does about 10 lights) and a little compound your in business.
This one is dirty now but still crystal clear i did this one over a year ago and its never been washed. This one was so yellow it failed inspection and had almost no light on the road.
I use this 5" orbital variable speed and this is the UV resistant clear, it gives very good results for those without spray equipment.
The disc, these are not a major brand but body shop supply quality. They are cheap but give outstanding results.
Finally if you have a spray gun and a respirator it doesn't get any better than this clear for durability and UV resistance. This is the original Dupont glamour clear formula. Also available in quarts.
The VW/Audi lights seem to go yellow fast on some years. They are an expensive light and worth doing. Aftermarket headlights are usually no where near as good a OE, you are better of saving the originals if you can.