Clearing a foggy headlight with acetone vapor - GIF

[JTG]

This is called vapor polishing -- when a plastic surface is exposed to the vapor of a solvent in which it is soluble, the surface will flow and small irregularities get smoothed out, which often brings the plastic back to optical quality. The solvent must be matched to the plastic for best results, and it is possible to overdo it and deform/craze the surface, though generally it's a pretty forgiving technique.

Acetone works well for polycarbonate, hence its use on headlights. This will only affect the outer surface of the plastic, and can give a surface roughness down to 10-15 microns; you can get the same or better results with mechanical sanding/polishing, which is what most of the headlight restoration kits involve, but it takes a lot longer. However, acetone vapor can wreck your paint, so masking the surrounding area is a good idea.

The technique here is to tape a (polypropylene/hdpe) funnel over the top of an aluminum can with a small amount of acetone in it, and then hold a heat source under the can to increase the vapor pressure of the acetone. If you're going to try this, it is critical that your heat source is not capable of igniting the vapor, and it should go without saying that you need very good ventilation.