Headlight polishing / clearing the yellow pitted plastic lenses and tailight polishing
Difficulty: 1/5
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Introduction
This article shows how to polish your dull yellowed headlight lenses to clear.  Glass headlight lenses are no longer used in non-sealed beam headlights due to safety regulations in the US.  The stock plastic headlight lenses will pit and oxidize with age and wear.  Polishing can make the lenses more clear again.  Keep in mind that stock headlights may also have a hardened surface.  If the headlights are worn down to the point where the damage goes beyond the hardened surface, then they are left more susceptible to road damage later on.  I suggest buying thin plastic headlight protectors to restore the protective layer.  

It's not just a cosmetic issue, it's also a safety issue for both you and oncoming cars.  Cloudy headlights diffuse the light and create glare for oncoming cars and prevent the light from reaching the road in a focused beam.

There are also headlight polishing kits that are available at auto parts stores.  A basic car detailing kit should already have the below materials since you will use them many times, but if you only want enough material to polish the headlights, the kits tend to be fairly priced.  Even still, you won't be able to get the same level of results compared to machine tools.  I also suggest the mothers powerball mini, it will make quick work of the final step and give you great results.  The full sized powerball are too large.

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Taillight polishing 
Taillights are not exposed to the same pitting but could still benefit from polishing.  You also 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.  The powerball mini should already have plenty of polish.  It'll make the colors "pop" more and clear up scratched plastic.  Don't use any sandpaper on them.

Parts and tools:

800 grit wet/dry sandpaper
1500 grit wet/dry sandpaper
2000 grit wet/dry sandpaper (optional)
mothers powerball mini

plastic polish - mother's plastic polish works well
microfiber cloth
painter's tape NOT masking tape or duct tape - any tape that is easy to remove and not 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.

 

Procedure

Start by taping off the paint around the headlights with painter's tape.  You can add an extra layer of protection if you want by adding another layer of painter's tape over that.  Don't use masking tape on the base layer because it can actually pull the paint off the car.  The first two pictures show how bad the headlights were to start.


The goal is to remove enough material to get down to a fresh layer of plastic that is smooth.  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.  If it's acceptable, use the 1500 grit, then the 2000 grit sandpaper if you want it really smooth.  Below are pictures after using 1500 grit.  I put a line of polish which was more than enough for both headlights (and the taillights).  A porter cable polisher is too large to use here, the powerball mini is just the right size and shape for the headlights.

The final result is that the headlights are much more clear!  One of the headlights was in the light which is why it appears more reflective.  Follow up with polish as necessary.  To prevent it 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.

Rough sanding
 

 

Light sanding

 

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

   

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