For permanent rust removal, the best method of ensuring it does not come back is to cut out the rusty metal and weld in a patch. Of course that gives you a butchered look, unless you or someone else is an artist with a MIG welder, grinder and finishing. Otherwise, sandblasting (with sand or some abrasize medium like aluminum oxide, not glass beads or crushed walnut shells)is the best way to remove the rust. It will be there, even when you cannot see it, if you merely try to wire wheel it or sand it out with a orbital sanding disk. After that, use a rust neutralizer (Rustoleum bought out the extend product line and has a blue aeresol can of something like naval jelly). I think PORS 15 is a rust neutralizer, available from the Eastwood catalog. There is one other popular neutralizer, but I forget the name. PUt on the neutralizer, wait 24 hours, then prime with an EPOXY primer (this is water resistant and adheres well). Then paint over the epocy primer with whatever you want your interior to look like. This will take care of the problem in most cases. Check that your seam sealer has not gotten old and hard, which means it can crack and allow moisture in. If you have the interior out, that is a good time to re-apply seam sealer, removing the old and re-doing. good luck!