Heavy surface rust treatment

[stangtjk]

I'm getting close to painting the body on my frame off resto and I'm paranoid about shooting paint only to have a rust bubble pop through a year down the road.

I had to put a couple patches in the quarters and in the inner bed side. As pretty much always the case these areas rusted from the inside out. Since there is no good way to get between the quarter and inner bedside i decided to remove the inner bedside so I could treat the back side of each.

Obviously these surfaces we're never painted from the factory except for the thin edp coating or whatever it was back then which would have been gone after a year or so. So naturally there is some heavy surface rust over the whole area. By heavy I mean some pitting but mostly just an even layer. The kind of stuff that you hit with a wire brush on a grinder and it knocks the loose dusty stuff off and leaves a glossy tightly adhered layer of rust behind.

The undercarriage and frame were blasted to bare metal but I don't really want to blast these parts so I tried the prep n etch rust remover (phosphoric acid). For the really light areas it dissolves the rust down to clean metal but for the other 95% it just turned the rust black. If you sand the black areas you eventually get back down to more rust so it doesn't seem to penetrate that far. From my experience this is pretty much the case for any of the rust "converters".

Anyone have any long-term experience with painting over these converted areas? Does the thin converted top layer effectively retard the rust underneath? I plan to epoxy prime and raptor line these pieces but not sure about the "converted" coating as I normally take it down to clean metal.