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I had the typical problem of yellow oxidized opaque headlights on my car. So I went through the process of wet sanding and polishing them and now they're crystal clear.

However, I've heard people say that the headlights will quickly ( within a few months ) oxidize and yellow again if I don't clear coat them, while I've had other people tell me clear coating them will only delay re-yellowing and later on when they need to be sanded and polished again the clear will just interfere.

So, what are the pros and cons of clear coasting my headlights after sanding / polishing them?

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  1. Headlights will yellow over time, almost like it's their purpose in life to do so. Once they do, you can treat them (as you have) or replace them.
  2. You will be sanding them again, when they turn yellow again. The clear coat, sitting on top of the plastic, will be removed by your sanding. Maybe it means you will have to sand a little bit more of the plastic off, but that's the extent of it.
  3. Speaking of sanding, how many times do you think you can sand your headlights before the plastic cracks for being too thin? Generally speaking, I expect headlights to withstand 1 good sanding.
  4. That sanding removes the most oxidised plastic. The rest of the plastic from that headlight is just as old, but it's been somewhat protected from damaging sunlight by the plastic that you've removed. It still has some oxidation, just not as much as the plastic you just sanded off. Get a full load of sunlight will make them oxidise to yellow much faster than a brand new headlight.
  5. The job of a clear coat is to be an extra barrier between what it is coating, be it paint or a headlight, and the outside elements, including the sun. A clear coat just might slow down the oxidising a little bit, but now you need to look at the cost/benefit. How much will all your supplies cost vs the cost of a new headlight.
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You can buy a non yellowing clear coat for plastics that will keep it from yellowing.

Video describing how to do it https://www.youtube.com/watch?v=UEJbKLZ7RmM

Non Yellowing Clear coat paint https://www.walmart.com/ip/Rust-Oleum-American-Accents-Ultra-Cover-2x-Gloss-Clear/37326872#about-item

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    Just to know that I used the Rust-Oleum and after a year it had peeled and yellowed and I had to re sand the headlights.
    – Robert S. Barnes
    Commented Aug 9, 2019 at 6:16
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Pros: your headlight lens will have an extra barrier of protection to protect the plastic against rocks, chips, cracks, and oxygen. It's cheaper to sand and coat a headlight than replace a headlight. $10 for a turtle wax headlight kit. $7 for a can of clear coat. Both of these products you can keep reusing for a lifetime.

Cons: it will eventually wear out, but what doesn't wear out?

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