How do I clear cloudy yellowed headlights?

Headlight lenses are polycarbonate with a thin ultraviolet-resistant coating. Once that coating fails, sunlight attacks the plastic itself and it clouds and yellows. Cleaning does nothing because the damage is in the material.

Safety

What you'll need

• Masking tape
• Wet and dry paper in several grades
• Polishing compound
• A UV-resistant clear sealant
• A drill or polisher with a foam pad (optional)

Step by step

1. Confirm the haze is on the outside.Run a wet finger over the lens. If it briefly looks clearer, the oxidation is on the outside and this job will work. Misting or moisture on the inside means a failed seal, which is a different repair.
2. Wash the lens and the surrounding paint.Any grit left under the masking tape or the abrasive paper will scratch the paint. Clean everything within a hand's width of the lens.
3. Mask generously.Several layers of tape all around the lens, covering paint, trim and any rubber. This is the step that separates a good job from an expensive one.
4. Start with the coarsest grade only if you need it.Heavy yellowing needs to start coarse; light haze does not. Starting too coarse means more work to remove your own scratches afterwards.
5. Wet sand with plenty of water.Keep the paper and the lens wet throughout, sand in one direction with light even pressure, and rinse frequently. The lens will look terrible and uniformly matt — that is correct.
6. Work through the grades in order.Each grade removes the scratches from the last, sanding at ninety degrees to the previous direction so you can see when the old scratches are gone. Skipping a grade leaves marks you cannot polish out.
7. Finish with the finest grade until the surface is uniform.It should look evenly frosted with no visible lines from any earlier grade. Any line still visible now will be visible at the end.
8. Polish to clarity.A cutting compound on a foam pad by hand or on a slow drill. This is where the lens comes back — it goes from frosted to clear over a couple of minutes per light.
9. Clean the lens with alcohol before sealing.Every trace of polish oil has to come off or the sealant will not bond. A microfibre and isopropyl alcohol until the cloth stays clean.
10. Apply a UV-resistant sealant or clear coat.This is the step people skip and the reason restorations fail. The factory coating is gone and bare polycarbonate yellows quickly. A dedicated headlight sealant or an automotive clear coat is essential.
11. Let it cure fully before washing the car.Follow the product's cure time, which is usually hours rather than minutes, and keep it dry meanwhile.
12. Check the beam pattern at dusk.Restored lenses put out considerably more light. Confirm the aim has not become a dazzle problem for other drivers.

Tips

• Do both lights even if only one looks bad. A restored light beside an untouched one is very obvious.
• Kits with a sealant included are worth more than kits with more grades of paper and no sealant.
• If the lens has crazing deep in the plastic rather than surface haze, a replacement unit may be the better answer.

Common mistakes

• Skipping the UV sealant — Sanding removes the factory coating along with the yellowing. Bare polycarbonate in British sunlight yellows again within months, often faster than the original did.
• Using toothpaste or a household abrasive — It may brighten a very light haze briefly, but it does not remove oxidised material and it leaves no protection, so the improvement disappears within weeks.

If it doesn't work

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