I've often wondered the same thing. You see the pro shops pulling roof skins off and the inner sheet metal has signifiicant surface (or worse rust). This is usually after the body has been media blasted and is sitting in their nice fabrication area. So I have always doubted that they go back in there and spot media blast it. The other thing that always had me wondering is they will often go to great lengths to show you all the nice patch panels they make and all the places they cut out any indication of rust in the cowls and rockers and glass frames, yet they never seem to address the inner stuff. From what I've been able to gather they seem to just wire wheel it and epoxy it and figure that inner rust won't progress far even in another 20 years and even so it won't bubble up and ruin the paint anyway.
So i'm no expert, but these days what I would do with that is just a quick wire wheel, and then rust convertor followed by POR-15. Accorind to the instructions you don't even need to use a rust convertor although I probably would since its a pretty simple step. Not all the reviews on POR-15 are glowing, but I"ve been using it quite a bit lately and have had really good success with it. I work in a carport and cosmetics and paint have never been my thing, so I often lose patience with prep or have to spary paint in the cold, but the POR-15 has been sticking and protecting everything much better than any spray can. I do have a gun and compressor, but its such a pain in the butt to mix up paint and clean the gun and throw away what I don't use I don't use it too much. Another nice thing about POR-15 is you can brush it on pretty quickly, no need to tape off much because there is no overspray and really easy to see how the coat is doing. Just don't drip it on anything pretty, its very difficult to get off including skin.
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