Usually it depends on the type of damage to the lense. I can describe three types of damage and how to repair them. Try to remove the lenses when possible and only if you have a way to secure them. If not pop the hood & use a durable masking tape 3/4in. & 2in. (auto paint and body tape) and apply atleast 3-4 layers so you dont sand the hood, bumper, & fender. Keep in mind when buffing headlights dont burn they melt so buff slow until you find a comfortable speed. In case they do melt don't worry try to wet sand then rebuff that area.
1. Just the yellow haze or film that has built up on the plastic. This is the easiest to repair
It is usually just a mild oxidation.
Such as the hazing on the Mercury.
Some times the haze will simply buff off with a heavy compound. If not wet sand it with 1500-2000 grit wet/dry paper. Followed by a heavy compound and polish. I like 3m perfect-it
any high speed buffer should work though I prefer 3in. wool and black waffle pads on a drill.
This should take about 1-1.5hrs. to complete
2. The second category is what I call pitted damage. This can be much more difficult to repair.
Note the Intrepid, 300M, Cherokee, & Mustang.
Alot of vehicles come with a claer sealer that is sometimes difficult to remove. Be sure to get it all off and sand deep enough to remove all of the pitts.
Begin wet sanding with 400 grit wet/dry paper
Next step up the sanding to 600 grit to remove the 400 grit marks
Continue this with 800- 1000- 1200- 1500- 2000 grit wet/dry sand paper
Now you can buff compound polish to remove swirls
This is where sealers come in but not an absolute necessity. Bright Solutions Shine Restorer will work or you can use Rustoleum clear spray paint ( not highly recomended) or clear coat ( expensive)
This usually takes 1.5-3hrs. maybe more in some cases
Tip for wet sanding. Alternate between grits with a vertical then horizontal motion so you can see that all marks from previous grit is completely gone
3. Water in the lense. This is usualy the result of an engine detail gone wrong.
The headlight must be removed. On the bottom near the back of the housing carefully drill a few holes with a 1/8in. drill bit. If water is in the housing pour a little bit of rubbing alchohol inside to help evaporate the h2o. Either vacuum or use compressed air to dry the lense. Seal the edge where the factory seal is with clear silicone from the hardware store. Do not seal the holes just drilled they will allow air to flow through and prevent more precipitaion build up. If silicone gets on the clear lense wipe it off with some rubbing alchohol.
Practice makes plastic lenses perfect