Why polished headlights turn yellow again in four months

Modern headlights are polycarbonate, which is tough and light and very sensitive to ultraviolet light. They leave the factory with a thin hard UV coating on the outside.

That coating is what fails. Once it breaks down, usually somewhere past year six, the plastic underneath starts oxidizing directly. Oxidized polycarbonate scatters light and turns amber, which is why the lens looks cloudy and the beam gets noticeably worse at the same time.

It is easy to file headlight haze under cosmetics. The measurable effect is on output: a hazed lens scatters the beam rather than shaping it, so less light reaches the road and more of it goes into the eyes of oncoming traffic.

On wet Northwest roads in December that is not a small difference.

Polishing abrades away the yellowed layer. The lens comes out clear, and for a few months it looks excellent.

What has also happened is that the last remnants of the factory UV coating are gone, and now bare polycarbonate is facing the sky. Oxidation restarts immediately and usually progresses faster than the first time, because there is nothing left protecting it.

Three stages, in order. Wet-sanding through progressive grits to cut past the failed coating and the oxidized plastic beneath it. Machine polishing to bring the lens back to optical clarity. Then a fresh UV-stable sealant to replace the protection that was removed.

That last step is the entire difference between a repair measured in months and one measured in years.

Some lenses are past it, and it is better to know before paying.