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A tip that I've seen on here is to have a clear coat painted over the headlights with plastic lenses in order to keep them from turning yellow.
I did this on my 1998 Dodge Dakota based on a youtube video by a guy in the N.E. USA who does restoration and detail work exclusively on
very expensive exotic cars. He said in the video that this is the proper way to restore plastic headlight lenses and I have no doubts about that.
Plastic headlight lenses are acrylic plastic and they are coated with automotive clear coat paint from the factory to provide a hard scratch resistant UV resistant coating to prevent UV degradation, oxidation and yellowing of the plastic (which occurs on the surface of the plastic). So, First you have to use sand paper to sand the old yellowed and scratched clear coat off of the acrylic lenses. WET SAND with water and 'wet sandpaper' down to a
fine grit sandpaper. It does not need to be extra fine, just the typical 'fine' grade. You can start with a medium grit to cut through the old coating and oxidized layers a little bit faster, then finish with the fine sandpaper. DO NOT USE A POWERED SANDER, at least not on headlights with curved and rounded surfaces because a powered sander will cut too deep and leave ugly machine sanding marks that won't match the contours of the lens. A disk sander will leave semi-circual sratches and it's very difficult to control the depth of the cut on these plastic lenses when using a power sander.
WET SAND BY HAND and you can use a sanding block on a flat lens. Any mistakes you might have made by using a powered sander will cover up very well when the paint goes on and will be very difficult to detect even on close inspection.
AFTER SANDING THERE IS NO NEED TO BUFF OR POLISH THE LENSES. That is a LOT of extra work and expense which is not necessary.
At that point (after sanding) the lenses will be totally fogged from the sanding. Then wipe them clean with a damp cloth and allow to dry. Mask them off and then just 2 or three very light coats of clear coat paint AND LIKE MAGIC they will become CRYSTAL CLEAR like brand new headlight lenses. The clear coat paint will protect the lenses from scratching, UV degradation and oxidation for a very long time. That's the way they came from the factory.
NOTE: It's probably a better idea to MASK OFF the headlight lenses before wet sanding. That will protect other surfaces from collecting the run-off of water, sand paper grit and dust.
I used s spray bottle to spray water on the lenses while sanding.
That method explains the principles of restioring headlight lenses. You have to remove the old deteriorated, scratched, oxidized and yellowed layers and then you need a good hard coating to protect the lenses from future scratches, UV and oxidation. There are LOTS of products that claim to restore headlight lenses. If they are good products they will accomplish the same things I've mentioned above, and probably most important is the final UV and oxidation protection. I don't know what they would use that is hard enough to protect from the scouring and scratches from sand while you are driving on the roads but clear coat paint does.
I don't know which paint formation is the 'best' for this application and would last the longest, eg, acrylic, enamel, or urethane - I would guess the urethane,
By the way, after I did the lenses on my 98 Dodge minivan night time visibility easily doubled, or better as did the visibility distance. It was nice to be able to see very clearly at night on two lane highways, in the rain etc.