I'm a believer in headlight restoration kits now
After doing some research, I found a kit that works.
Two things up front.
First, when I say "headlight restoration", I actually mean "headlight lens restoration", because it's the lens you want to restore since the bulb can be replaced (assuming you're allowed to do that on your car).
Second, this is the before photo so you know what I was working with:
The kit I bought and used, which comes in a rather small and unassuming box, is the Sylvania Headlight Restoration Kit.
The reason the box is on the smaller side is because it comes with no drill attachments. Almost everything you need is in the box. The only two things you need to provide are water (in a spray bottle, bucket or from a garden hose) and paper towels. Since there are no attachments, all sanding must be done by hand...
...which is exactly what I did.
While true there are many kits out there, I went with this one primarily because the sandpaper provided isn't garbage, and the cleaning solutions provided are better compared to other kits.
Yes, there are other kits that come with the drill attachment with sanding discs. However, as convenient as that sounds, where a lot of those cheap out is with the sanding paper. If that paper isn't the good stuff, all the convenience by using a drill attachment is lost right out of the gate.
With hand sanding you don't splatter solutions all over the place, but I will readily admit that it does take longer to get the job done.
How much time does it take to use this kit? About 30 minutes per lens if you've never done it before. All the prep, sanding, polishing and the final UV coat takes about that long. If you have done it before, then it's 20 minutes per lens.
With the Sylvania kit, all you have to remember is this: If you follow the instructions EXACTLY as given, it will work. The best way to go about it is to follow steps 1 and 2 for both lenses first, then step 3 for both lenses after that. If you get this kit, you'll see what I mean when you read the instructions.
Other things I know work but really don't when it comes to lens restorations
A lot of people say toothpaste and/or baking soda works for headlight lens restoration.
Does this work? Yes, but only for light clouding, and it's guaranteed the lenses will cloud up again quickly. Why? Because the lenses weren't coated/sealed afterward. Headlight lens sealer is cheap, and yes, this stuff is required. It's better if you get headlight clear coat. In the Sylvania kit, the sealer/clear coat is the UV block clear coat for the final step.
Both toothpaste (as in the whitening type that has silica in it) and baking soda when rubbed over a lens acts as a very light abrasive, so yes, it does work. But wow, do you have to rub-rub-rub for a long time just to get either to do anything. You might get moderately okay results, but again, the lens will cloud right up again quickly.
Can this be done cheaper?
Sort of, but it's not really worth it.
If you were to part out everything in the Sylvania kit (400/1000/2000 sandpaper, microfiber towels, cleaning solution, rubbing compound and clear coat), it would be all too easy to spend more than double what the kit costs.
Many guys already have the sandpaper and drill attachment. It's the solutions where you rack up the cost. Just the rubbing compound and clear coat can easily cost more than double the price of the kit.
The absolute most dirt cheap way to do it for really clouded up lenses is to use water and 400-grit sandpaper for 5 minutes, repeat with 1000-grit for 5 minutes, repeat again with 2000-grit for 5 minutes, then use UV clear coat wipes from a 3M Quick Headlight Clear Coat kit. The kit alone isn't enough because it only comes with one sanding disc, hence why you need to provide the 400/1000/2000 yourself in addition to that kit.
With the sandpaper and the kit, you can get the cost down to under 20 bucks. Worth it to save a few bucks over the Sylvania kit? Not really, because you barely save any cash.
Prior to using the Sylvania kit, I never restored headlight lenses before and the results I got were decent.
How long do I expect my clear lenses to last?
The best way I can answer that is that I don't expect miracles here. If I get a year or two out of this before having to do it again, so be it. There's just no way a small kit can match the OEM baked-on coating the auto manufacturer used originally, nor am I expecting it to.