See the deer before the deer sees you.

It is a safety problem before it is a looks problem

Modern headlight lenses are polycarbonate, not glass, and they leave the factory with a UV hard coating on the outside. Sun degrades that coating, and once it fails the plastic underneath starts to yellow and haze.

What that costs you is light on the road. A hazed lens scatters the beam instead of projecting it: the bulb is as bright as it ever was, and less of it is landing where you are driving. On a two lane road out of town at night, that is the margin you have to see something in the ditch.

Sand, polish, seal

The failed coating and the yellowed layer under it have to come off, and that means wet sanding through progressively finer grits until the surface is uniform. Then it gets machine polished back to optical clarity: a lens that is merely 'clear' to the eye but still finely scratched will still scatter light.

Then it gets a UV sealant, and that stage is the whole reason to have it done properly. Both lenses, same visit, because one restored headlight next to one yellow one looks worse than two yellow ones.

Why the kit from the parts store goes yellow again

Every drugstore restoration trick (the toothpaste, the bug spray, the $20 kit) does the sanding and polishing half and skips the UV seal. So the lens looks great for a month and then yellows faster than it did the first time, because now there is no protective coating on it at all and the raw polycarbonate is exposed directly to the sun.

That is not an argument that those kits are fake. It is an argument that the durable part of the job is the part they leave out, and in this climate the sun finds it within a season.

When a lens cannot be saved

Yellowing and hazing live on the outside and come off. Crazing on the inside of the lens, moisture trapped inside the housing, or a cracked lens are different problems, and sanding the outside does nothing for any of them. We will tell you when what you have is a housing replacement rather than a restoration.