As far as I know, headlights and windshields are made of either one of two plastic resins.
The first is acrylic which goes by the trade names like Plexiglas or Perspex. The second is polycarbonate which is commonly trademarked as Lexan. Both have excellent optical properties, and are impact resistant which are important for these applications. While both are fairly tough, they are soft and suffer impact and abrasion damage.
To protect the relatively vulnerable surfaces, manufacturers treat them with a protective coating. I don't know what that coating is, but I would suspect it is a polyurethane based clear coat similar if not identical to what is put on the painted parts. Polyurethane is very tough and has amazing abrasion resistance. The downside is that it is not very stable under the ultraviolet radiation in sunlight. Even with UV stabilizers it will break down over time, clouding and yellowing, and accumulating some scratches. The clear is so tough that normal plastic polishes like Novus aren't aggressive enough to have much effect.
I have often thought that removing the OEM coating and replacing it would be a reasonable way to renew a windshield. I just wasn't sure how well the clear coat would cover the scratches from the sanding used to remove the old coating. The video seems to indicate that re-coating is a viable way to restore the optics of a windshield. Since I have an old windshield here that I am thinking of selling, I think I will give it a try on it and report back my results. Only caveat is that it probably will be a while before I can get to it.
