Just because it doesn't make a car faster doesn't mean it's not worth testing at SCC. In this case, the GlassyLite headlamp restoration and protection treatment will be of interest to nearly every reader, since modern cars use molded plastic headlight assemblies that are lighter, cheaper, safer and easier to mold into complex body shapes.
Gone are the days of complete replacement headlamps or glass units that never had issues with hazing or yellowing. The problem with a polymer-based headlamp is its tendency over time to degrade in transparency. This comes from two sources, hazing and yellowing.
Because modern headlamps are made of softer polycarbonate (PC), trademarked as Lexan, a hard acrylic outer coating and UV protectant has to be applied by the OEMs at the time of manufacture. Although harder, the very thin acrylic (trademarked as Plexiglas) surface layer will still scuff and wear from accumulation of scratches from washing and road grime impact. This is hazing and, aside from deep debris impacts, it's a surface effect and light is defracted through these scratches.
Yellowing is a chemical and environmental reaction to the heat, UV, and gamma radiation a lens is always exposed to. As the exterior acrylic layer is worn away, yellowing will begin within the lens. As with any plastic, heat from the light bulb and sunlight will begin to break down the molecular bonds within the polymer chains. This is why the portion of the headlamp directly exposed to the halogen beam is often the most yellowed: it gets double exposure, from the sun on the outside and the beam inside. If a headlamp is already beginning to show signs of yellowing, then the hard acrylic protective coating on the surface of the PC plastic has gone. In order to eradicate this haze, the damaged surface has to be sanded to remove the scratches and embedded dirt.
Several headlamp restoration products do just that. The haze is buffed out with a rubbing compound, leaving the polycarbonate completely unprotected. But once that OEM protection is gone, there is little to prevent additional yellowing or hazing of the now-exposed softer PC lens. A small percentage of products will also include a wax-based covering to help restore a smooth, wet look, but this can disappear after just a few washes. A few other products use a clear lacquer or urethane coating (automotive clear coat) sponge or brush applied to the surface of the lens. These materials work, but tend to leave streaks and they also yellow over time, like any paint.
We tried a new product, developed in Japan and imported by Tourenn, called GlassyLite Automotive headlamp restorer and protector. This is a DIY kit that can be used on brand-new headlamps or to restore torn-up old ones. It consists of 1500- and 3000-grit sandpaper (for wet-sanding away the damaged OEM PC surface), a specialized cleaning agent and an aluminum oxide-based rubbing compound for buffing out the haze from color sanding. Not too different from other comparable products.