Look at it this way. I subframed my frame. That is a boxed or tubular frame welded to a "C" channel frame. Opening that tubular frame to coat the inside just isn't an idea that I would consider or an idea that would be smart. Rust will form then the rust will slow. Coat that parts as best you can, get into all the nooks and crannies as best you can and don't worry about the sandwiched steel. Unless, of course, your rivets are loose. Then you will want to address that problem.
