Polycarbonate degrades under ultra violet light, so wherever it's used to make items that will get a lot of sunlight, it's protected with a stabiliser. The stabiliser is a compound that readily absorbs UV with little apparent effect. The stabiliser may be mixed into the molten polycarbonate prior to moulding or extrusion (eg roofing sheets) or applied as a separate layer, for example by spraying on. Headlamps are coated like this and the stabliser prevents UV degrading/embrittling the lens. The layer is sacrificial and eventually becomes itself degraded. That's what you abrade away during the headlamp treatment. The yellow layer can sometimes be seen quite distinctly during the abrading process. If the stabiliser isn't replaced, the lens itself will begin degrading immediately. In my experience, a re-polished but uncoated lens will become cloudy in 24 - 36 months with the car parked pernanently outside. UV stabiliser treatments have got a lot cheaper in recent months. I'd recommend anything that states 'UV' in the name or the information on it. Those finishing wipes that applies a film from a moist tissue will last from a few days to a few weeks at best.
TC
Last edited by T-Cut; 8th August 2019 at 08:09..
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