I sandblasted the frame that's now under my 70 Parisienne, and rattle can primed and painted it. Sandblasting doesn't touch the inside of the boxed frames, so the surface rust can still be seen if looking for it.
I do my impala every 2 years with rust check.
supervise the spraying so that the frame rails get lots inside. Focus on front rails and rear hump over the axle.
everything else gates a coating.
That's a good way to do it - I wouldn't have thought of that.
Krown/Rust Check is great stuff for protecting inside of frame members etc. I do all my cars with it, even the one that is stored during the winter. It creeps into all the little crevices that even paint can't get to.