Headlight polishing / restore the foggy yellow pitted plastic lenses to new and tailight polishing
Difficulty: 1/5
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Introduction
This article shows how to polish and clean your foggy dull yellowed headlight lenses to clear.  Glass headlight lenses are no longer used in non-sealed beam headlights on most US cars.  All plastic headlight lenses will pit and oxidize with age and wear.  Polishing will restore their clarity and make the car look much better.  And it's not just a cosmetic issue, it's also a safety issue for you and oncoming cars.  Cloudy or hazy headlights diffuse the light and create glare for oncoming cars by scattering the light.

Headlight lenses may also have a hardened surface.  If the headlights are worn down to the point where the damage goes beyond the hardened surface, they are left more susceptible to road damage later on.  Once you polish the headlights, I suggest buying thin plastic headlight protectors to restore a protective layer or doing some follow up polishing as maintenance.

There are headlight polishing kits available at auto parts stores.  A basic car detailing kit should already have the below materials but if you only want enough material to polish the headlights, the kits tend to be fairly priced.  Machine tools are much much faster and produce better results so I strongly suggest a mother's powerball mini and plastic headlight polish.   The powerball mini attaches to the end of a drill.  The full sized powerball or porter cable polisher pads are too large and the powerball for paint is too soft.  3M also sells a headlight restoration system but the polishing/sanding pads are relatively flat - the powerball mini is a ball and can be used on curved surfaces.

Taillight polishing 
Taillights are not exposed to the same pitting but could still benefit from polishing.  You use the same materials so it only requires a few more minutes.  After you're done with the headlights, use the mother's powerball mini on the taillights and side corner marker lights.  It'll make the colors "pop" more and clear up scratched plastic.  Don't use any sandpaper on them.

Parts and tools:

800 or 1500 grit wet/dry sandpaper (for heavily yellow headlights only)
2000 grit wet/dry sandpaper (for lighter oxidation)
mothers powerball mini or 3M headlight restoration system or meguiar's headlight restoration system
plastic polish - meguiar's plastx polish or Novus2 works well (if not included in the above kits)
microfiber cloth
painter's tape - not masking tape or duct tape - any tape that is removes easily and cleanly and isn't too sticky
small handheld sander approved for wet/dry use - don't get the sander wet unless you know it is approved for wet/dry use or else it could result in serious injury, death, and/or property damage from among other possible causes, electrical shock. (optional)

The porter cable polisher is pictured because I used to use it - the powerball mini is much faster and easier to use and is the right size.

Here are some video demonstrations of the meguiar's and 3M system in use.  Below are more pictures and details.
 

Procedure

Start by taping off the paint around the headlights with painter's tape.  You can add an extra layer of protection by adding 2 layers of tape.  The first two pictures show how bad the headlights were to start.

In these cases, the headlights were so pitted and cloudy that they needed to be sanded down.  If the headlights are in fairly good condition, just use the powerball mini and some plastic polish.  Move in a crosshatched and diagonal pattern to cover the entire lens.  That should be enough to make the lens clear.

If that isn't enough or the headlights are too far gone, you have to use sandpaper.  The goal is to remove enough material to get down to a fresh layer of plastic that is smooth.  The worse the headlights, the more material you have to remove.  If the headlights are very bad, start with 800 or 1500 wet/dry grit sandpaper depending how opaque the headlights are.  If they aren't too bad, start with very light 2000 wet/dry sandpaper.  The headlights should appear cloudy but uniform.   Use some elbow grease here!

Rinse off all the slurry and determine if you need to sand some more.  I put a line of polish which was more than enough for both headlights.  A porter cable polisher is too large to use here, the powerball mini is just the right size and shape for the headlights.  Push the polish into the powerball and spread it around to avoid flinging and wasted polish.

The final result is that the headlights are much more clear!  One of the headlights appears more reflective because it was in the light.  Follow up with polish as necessary.  To prevent the lenses from getting pitted again, use plastic film approved for automotive use to protect from future damage or specially designed headlight protectors.  If you buy a new car, hopefully you will have applied this knowledge and get headlight protectors so it will never need polishing in the future.

The powerball mini can also be used on the turn signals or side markers and rear taillights.  However, since the taillights are not bombarded with road debris, do not use sandpaper on them, only use polish.  After polishing, the colors will appear much more vibrant and "pop" more due to the smoothed plastic.

Here is another example of headlight polishing on a really bad case.

Rough sanding
 

Light sanding

Final polish results - no camera tricks, just clear lights!  

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