I don't see how any machine could be powerful enough, hot enough or whatever enough to remove 1/2 inch thick undercoating but somehow not remove inspection markings.
I see at one of the posts above, they recommend softening the undercoating with gasoline. Our regs are pretty minimal here in GA but using gasoline as a chemical solvent is one thing that we can't do. We don't screw around with the EPA here. The guy we bought the shop from had such heavy EPA fines that the government got all of the money from the sale.
Dry Ice Blasting Principles
Cleaning with dry ice! This new development is quickly expanding around the world. One system uses small rice size pellets of dry ice shooting them out of a jet nozzle with compressed air. It works somewhat like sandblasting or high-pressure water or steam blasting, with superior results. The frigid temperature of the dry ice -109.3°F or -78.5°C "blasting" against the material to be removed, causes it to shrink and loose adhesion from its sub surface. Additionally when some of of dry ice penetrates through the material to be removed, it comes in contact with the underlying surface. The warmer sub surface causes the dry ice to convert back into carbon dioxide gas. The gas has 800 times greater volume and expands behind the material speeding up its removal. Paint, oil, grease, asphalt, tar, decals, soot, dirt, ink, resins, and adhesives are some of the materials removed by this procedure. Only the removed material must be disposed of, as the dry ice sublimes into the atmosphere.
