Why do restored headlights turn yellow again?

Headlight lenses are polycarbonate plastic protected by a factory UV hardcoat. Restoration usually sands or polishes away the oxidized hardcoat, leaving bare polycarbonate that yellows quickly in sunlight if it is not protected again. After restoring, apply a dedicated UV-resistant headlight coating or clear coat designed for lenses, and maintain it. Simply polishing and leaving the lenses bare often lasts only a short time.