the windshields on a vehicle are all made of laminated glass, two separate pieces held together with some sort of polyvinyl material, not sure of proper name. When it gets a chip it is usually only in the surface layer. When the repairs are done the chemical reaction is with the center layer of poly, which allows the patch to work when it is heated. A visible cylinder is nothing more than a round piece of single thickness plate glass. when a repair is attempted there is nothing to bond with so it will probably never work. I say "probably" because I never say never. When I was restoring pumps I usually considered myself lucky if the cylinder only had a couple of BB cones in it. Norm Huff
