This is what I believe is the main cause of oxidation build-up on your headlights.......
Ok, so a (UV) coating is applied to your factory Auto headlight lenses to help protect these newer acrylic / plastic headlight and taillight lenses from the elements as well as the suns harmful (UV) rays.
It took scientific processes, testing and retesting to create these new Acrylic / Polycarbonate lenses as well as the (UV) coating that is applied to them.
In time this UV coating develops a layer of oxidation over the surface, a combination of oil, road grime and other air borne contaminants is what the oxidation is made up from and is cooked onto the lenses from the heat of the lights.
I believe this because in most cases I've seen if a car has turn signal lenses that are separate from the headlight lenses the turn signal lenses will have little or no oxidation.
This TOYOTA shown above is a perfect example of what I'm talking about.
NOTE: The top image, separate lenses. This is a before & after I restored the headlights, but I didn't do anything to the side lenses.
So I thought, why is the headlight so badly oxidated yet the turn signal looks great!
Well the sun should be shinning equally on both lenses and be heating up to the same temperatures without the lights being on.
The lights are not in the same housing, the turn signal and park lights do not get as hot as the halogen headlights do and the headlight casings are not vented near enough to disperse so much heat.
So I believe that most oxidation build-up occurs when your headlights are on, especially for long periods of time, Day or Night.
Eventually this oxidation will damage your factory UV coating and resurfacing them will be your only option. So it is best to have them de-oxidized in the early stages of oxidation. Before it starts to distort your headlight beam and range of view. By Scott Summerson
Hello, Scott Summerson here, Here's a little about my experience: