How to polish the headlights and restore the foggy pitted plastic lenses to clear
Difficulty: 1/5
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Introduction
This article shows how to polish and clean your foggy dull yellowed headlight lenses to clear.  I also review the 3M headlight polishing kit.  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.  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.  HID lights will have unacceptable dazzle if the lenses are not clear.   

Here is an example of before and after polishing.  The city light (the small parking light) and its reflection are no longer hazy and there's a lot less scattered light and glare from the foggy lens.  Less glare from clear lenses means more light onto the road.  The picture angle is slightly different but the results aren't a camera trick - it's clear the amount of glare and lens fogginess is a lot less.

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 or vinyl headlight protecting film so that you don't have to polish them again for a while.  Professional installation of the film is suggested.  The other option is to do a final polish once a season to keep them clear.

There are headlight polishing kits available at auto parts stores but I suggest using the ones that use power tools.  Power tools are much faster and produce much better results than hand polishing.  I strongly suggest a mother's powerball mini and plastic headlight polish.  Even if you do buy the polishing kits I still prefer the powerball mini over their foam polishers.  Its rotation and contact surface is more consistent, easily controlled, and will give better results because of the many fingers and attaches to any power drill.  The full sized powerball or porter cable polisher pads are too large and the powerball for paint is too soft.  

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 unless they are extremely bad.

3M headlight restoration system review - it's a good value if you find it on sale or have a complex curved headlight.  The foam pad follows sharp curves better than a flat electric sander.  Here is what's included in the 3M headlight polishing kit: headlight polish, polishing pad, disc attachment for the power drill, 3000 foam pad, 800 sanding disks, and 500 sanding disks.  

I found better and faster results using a power sander and a mothers powerball mini.  The main problem is that the 3M kit's 500 sandpaper is pretty rough and the 800 seems soft.  There's twice as many 500 sanding disks than you would ever need and too few higher grit.  After the 800 they go up to 3000 pad and a polishing pad that is too soft.  I found it left swirls because the change in grit is too great.  If you buy the 3M kit avoid using the 500 grit paper and start with the 800.  I also still recommend a powerball mini if you buy the kit because it works better.  The 3M pad rotates as a flat pad perpendicular to the drill's axis.  The outside diameter of the pad moves much faster than the inside diameter of the pad which is barely moving at all.  The powerball mini rotates parallel to the drill's axis and gives more consistent force.  The powerball also has many foam fingers that polish each stroke vs. the 3M foam pad which uses a flat surface.

My recommendation on parts
Because the 3M kit is designed to be sanded dry and because of the big change in grit, I recommend putting together your own kit.  The 3M kit works well if you start with 800 and use a powerball mini.  It's also good if you are only going to sand the headlights once and the only tool you have is a power drill.  I've tried it a few times and the best and fastest results I've gotten are with a DIY kit.  I recommend a power wet/dry sander starting with 800 grit wet/dry sandpaper, then 1500 wet/dry, then the powerball mini with plastic polish.

Pictured is my DIY kit - plastX plastic polish, a powerball mini, and a wet/dry electric sander

Parts and tools

3M headlight restoration system or meguiar's headlight restoration system if you want to buy a kit

OR

800 grit wet/dry sandpaper for the first stage and 1500 for the second stage
small handheld sander approved for wet/dry use (optional) - 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. 

mothers powerball mini
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

Here are some video demonstrations of the meguiar's and 3M system in use.  Below are more pictures and details on how to polish foggy lenses.  The 3M headlight polish system works well on highly curved lenses but after using it I still prefer a flat sander and powerball mini.  The 3M kit with a powerball mini works best for curved lenses.
 

Procedure

First, is the inside of the lens clean?  Some projector headlights tend to get dusty on the insides.  Use a grabber tool like craftsman 41322 and wrap the end in tape to prevent scratches.  Then use the grabber to hold a clean microfiber cloth.  You can also use a lint free cleaning cloth.  Make sure the metal grabber fingers are covered and wrap the grabber in tape if you are concerned about scratches.  Put a small amount of no residue window cleaner on it.  Remove the light bulbs and stick the grabber tool through the bulb holes to wipe the lens clean from the inside.  You shouldn't be able to see through projector headlight lens but you should be able to tell if it's dusty or clean.

Then wash the exterior of the lens.

If the headlights are in good condition without fogging, just use a powerball mini and some plastic polish.  Move the polisher up-down, left-right, crosshatched and in a diagonal pattern to cover the entire lens.  That should be enough to polish the lens.  If you have the 3M kit, just use the 3000 gray pad (softer than sandpaper) and follow up with polish.  If there are visible pits or fogging you'll have to sand them down as described below.

Start by taping off the paint around the headlights with painter's tape.  

The headlights below needed stronger sandpaper in the first stage to remove the surface defects and the yellowed oxidized layer of plastic.  Don't spend too much time with 500 grit since it's pretty rough.  If you are putting together your own kit I would start with 800 sandpaper.  You can see the final result with these lights at the bottom of the page.

If you're using a flat electric sander, move it back and forth, up and down, and diagonally.  Wet/dry sandpaper is supposed to be wet and form a slurry as you sand.  This prevents the sandpaper from getting loaded.  If you are using the 3M pad, hold it at a slight angle to the surface instead of perfectly flat to the surface.  This puts the sanding surface over one edge and gives better contact control over the pad.  This is because as the circular pad rotates, the center moves slower than the outer diameter.  Don't pause in any spot too long or else it will burn the lens.  If the pad is getting loaded then brush it clean.

This headlight isn't as foggy and only needs 800 grit to smooth it out.  Use the minimum grit required to smooth out your lens.  Any stronger will result in swirl lines and scratches later on.

If you see wavy lines after sanding, the surface of the lens isn't completely flat.  The wavy lines are from the high spots getting sanded down and the low spots not.  Go back and sand them down.  The headlights should appear cloudy but uniform.  Rinse off the slurry and determine if you need to sand some more. 

Use a higher grit sandpaper at each step.  The 3M kit tells you to use 800 grit, then the 3000 foam pad.  If you are using your own materials I would suggest using 800, then 1500.

Examine the lens - there should be no wavy lines.  If there are, go back and sand some more.  Once it's smooth and uniform, polish the lenses.  

The powerball mini is just the right size and shape for the headlights and I prefer it over the pad provided in the 3M kit.  Push the polish into the powerball and spread it around to avoid flinging and wasted polish.  Move it up/down, rotate the powerball rotation axis and move it around left/right, rotate it again, and move it around again.  

The final result is clear headlights!  To prevent the lenses from getting pitted again, use UV resistant plastic film to protect from future damage or specially designed headlight protectors.  These are difficult to apply without bubbles or defects so professional installation is recommended.  If you buy a new car, get headlight protectors so it will never need polishing in the future.  At some point the headlight protectors will wear down but you can just peel off the old layer, wipe away any adhesive, and apply another protector.

The powerball mini can also be used on the turn signals or side markers and rear taillights.  Since the taillights are not bombarded with road debris, you'll only need to polish.  After polishing, the colors will appear much more vibrant and "pop" more due to smoother plastic.

Here is another example of headlight polishing.  Rough sanding with 800 grit.
  

Light sanding with 1500 grit.

Final results:

 

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