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So a year ago the headlights on my ten year old car started going yellow and opaque. Ive often heard you can clear that up with toothpaste. So I gave it a shot and it worked perfectly. My headlights were crystal clear and smelled minty fresh. However, about 6 months ago they started fading again, so I cleaned them again. And once more, they are starting to fade. I will scrub them again.

However, perhaps there is something you can apply that acts as a sealant to make them stay clean longer?

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As @Zaid said for the reasons for yellowing/clouding is spot on: UV rays. As for the fix, I would do it slightly different.

First, you can use toothpaste to clean off the lens. This works to a degree, but to do the proper fix, take the wet sanding approach. Start by using a heavier grit wet sand paper, then use progressively finer wet sand paper. The idea is to get rid of the yellow, then get rid of the marks left behind by the coarser sand paper.

With that in mind, you'll next want to protect your new covering. By using a UV protected clear coat out of a spray can, you can completely cover the lens (even the parts which may be hidden) and not have to worry about it for another 5-8 years. The reason to use sand paper instead of toothpaste is the spray needs something to hold onto. The fine scratches from the sand paper will do that for you. When you spray the lens, you want to ensure you put several fine coats on it and not just one thick coat. Put the coats on about 5 minutes apart. You want it to start to dry and still be tacky. If it isn't tacky, the next coat will not stick to it and will most likely peal off after a relatively shorty period of time. There are many brands of the spray, but here is one from Krylon which should give you an idea of what you're looking for.

I've watched this video which is a great run through on how to fix the issue for a long time to come. The guy says it's a "permanent fix" ... the jury is still out on that, but it will fix it for a long time to come. He goes through it step by step as I basically described above.

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    +1 This. I prefer having a clear coat spray over a piece of film over the lights; it seems that having the spray keeps the headlamps clearer longer – yuritsuki Oct 10 '15 at 18:02
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Yes there is, but first a little story to explain why the headlights are going yellow/cloudy so quickly.

Any plastic headlight lens will yellow through exposure to UV radiation (which the Sun happens to have buckets of). Brand-new plastic lenses from the factory will invariably have a protective coating that is designed to deflect away much of the UV radiation from the actual plastic surface itself, which considerably slows down the rate of yellowing.

When headlights are deemed yellow/cloudy/opaque enough to warrant action, most people choose to polish up the surface, which does a decent job of removing the damaged surface. Unfortunately, it also removes the protective UV coating, and many people neglect to do anything to replace it. It comes as little surprise then that the yellowing returns with a vengeance.

You can get the headlights coated with a clear, protective film that is designed to protect the surface of the headlight lens from pitting and scratches. The film is installed after the headlight lens is polished, so I'm not sure whether the UV protection comes from the film or the wax that is sealed in, but this combination helps the lens hold up remarkably well against yellowing.

There are many vendors out there. A quick web search with the terms "headlight protective film" should get you something for your specific vehicle quite readily.

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