Polycarbonate and its sacrificial layer
A headlight is a sealed optic: the lens must stay perfectly transparent for the reflector behind it to project a clean beam. On models from before the 2000s that lens was glass — inert. Polycarbonate replaced it for safety: it bends instead of shattering at the first pebble. That choice has a cost every owner pays around year five: the surface yellows, and the beam crossing it loses its edge.
Over the summers, UV and wash abrasion mine that clear coat. Once breached, the polycarbonate oxidizes at the surface: it turns from clear to yellow, then milky, and the headlight beam scatters inside the hazy layer instead of passing through. The process is slow — three to eight years depending on sun — but irreversible: no product repaints oxidized plastic. The only path is mechanical: strip the affected layer, then reseal.
What speeds up yellowing in Montreal
The first accelerator is outdoor parking: a car sleeping in street-parking sun ages its headlights twice as fast as a garaged one. The second is salt and brine: five months of winter where spray dries on the lenses and micro-cracks the weakened clear, opening doors for UV. The third is abrasive washing: every brush pass or dry towel removes part of the protective layer.
Heat finishes the job: a lit headlight heats up, and a headlight heating in the July sun of a south-facing lot works its layers constantly. Black and dark cars hold more heat and often show tireder headlights at equal age. The cheapest lever remains shelter: even a windshield sun shade slows the process, and an intact clear coat lasts years longer than a breached one.
What restoration really involves
A proper restoration is sanding, not a product. The lenses are removed or the body masked, then the polycarbonate is sanded in progressive grits — typically 500 to 3000 — until every yellowed layer is gone and the surface is uniformly matte. That is the step pharmacy kits skip: without full sanding, oxidation stays trapped under the sealant and returns within weeks.
Polishing then restores clarity, followed by the decisive step kits forget: UV sealing. Sanded polycarbonate is bare, without its original protection; left as is, it re-yellows within one summer, faster than before. A sprayed UV clear coat cured properly, or a dedicated sealant, closes the surface for two to five years. A full restoration takes one hour for a pair of headlights, car included.
When to restore, when to replace
Restoration recovers surface oxidation: yellowing, milky haze, lost reach. It does not recover everything — a cracked or punctured lens, or one with permanent inside condensation needs replacement, because the problem is no longer on the surface. Likewise, a lens sanded twice without proper sealing no longer has enough material for a third pass: replacing the lens or the assembly is the better call.
The right time to restore is before inspection and before fall: a beam cut by an opaque lens loses up to 40 % of useful reach, and night comes early in November in Montreal. After restoration, two habits extend the result: washing the lenses with the rest of the car to strip salt, and reapplying a clear or sealant as soon as beading disappears — the sacrificial layer is back on duty, it wears, it gets replaced.
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