one local company will coat the inside with "box liner" thickness and then spray the outside in a thinner coat. We shot the side of a Jeep that was coated with this with a 12 guage shot gun at about 25' and it didn't make much of a mark.....he tried the slug after but that is another story and that did leave a mark.....left some glass in the bottom of the door and a hole in the seat.
I'm not sure about the stuff you've got but the porfessional box liner stuff was a pain if you wanted to weld in some panels or need to get to the metal to do some work.