David, you missed the point with City Hall. You see, I was a professional automotive glazer for 17 years, branching out into theft recovery and chassis later in my career. The difference between commercial glazer and automotive glazer is that automotive glazers can do commercial, but commercial cannot do automotive because it takes a lot more mechanical reasnoing to deal with some of the issues in related automotive components.
City Hall meant the norm, not regulations. As a pioneer in structureal automotive glass and developing effective salvage methods, I definalty voilated rules set forth by city hall, saying that the glass cannot be removed without breaking it. Even GM Service manuals stated: "cover glass with masking tape and break glass to remove."
Done only as a courtisy for better customers, it's a safe bet to say that I have buffed scratches out of multi hundered windsheilds, some with exceptional results, and I have taken many stabs at scratched tempered even though it was said that it couldn't be done. This is a case of city hall being correct.
And it has nothing to do with altering the strenght of tempered glass. Plate molecules are malable and aligned, allowing for clean easy fractures. Tempered glas has its molecules reorgized so that they are in a randon, crystal-like interlock. This bind allow each molecule to be sympathetic for its neighbor, clingint tightly. As a reslult it takes epxonential force to cause a fracture, but when the fracture occurs, the molecular chain become disrupted throughout the entire glass, the reason it becomes gravel when it breaks.
A scratch or a chip doesn't affect the mollecular chain. I love the niave cop syndrome when you see them on tve beating on tempered a glass with a steel flashlight. You would think that part of their job would involve understanding the mocular chain and how it interacts. Theives do because they can pop tempered in a nanosecond, with it barely makeing a sound, and it's done with just a slight flick and little applied force. Unless it's a catrostropic blow--to withstand blows similar to those delivered by the police departmet--it's hard to break. You must send a shock wave between indivigual molecules, and it doesn't require much of a shock wave if it's targeted properly. And slight variances in thickness has little bearing on strenght.
Maybe there is a way to cut tempered, but it would be a grinding--eliminating material on the mocular level--but not a traditional score and snap process
Even if there is methods in buffing flat glass--eliminating material in a molecular level--it wont work with curved glass because maintaing optical clarity becomes an issue. (Vent windows, I belive, fall into the flat glass area.) But rather than getting into an optical clarity dissertation, the point is that buffing scratches out of automotive temper--which is curved glass for almost all automotive applications--doesn't happen.
In addition to the quests throughout my professional career, the buffing tempered topic has gone on and on and on for years and years here at the board. People have posted here asking about the assorted weasel p!$$ on the market. The comments become redundant, light scratches in a w/s but temered is a waste of time and money. And the feedback for w/s scratches range from came out excellent (if ever so slight)it's better than before, or the area is not longer scratched, but there is a bid distored area. As far as tempered, the reports are the same: It didn't do a thing.
If you think that you can buff out the scratches, because somone tells you that you can do it, go for it. I can shift into the don't give a dam what others say, the reson I know that scratches cannot be buffed out of tempered automotive glass.
