I've been working with the rust on my van this weekend, and I believe that if
you do not get to the rust from the underside (inside) then there is no fix
other than cutting it all out as others have suggested.  If you can to the rust
from the inside, then you can do the sand/rust-converter/primer etc. from
both sides and it may last a while.   Also while it is apart, be sure to spray
rust inhibitor oil all over the inside of the panels.  Malcolm S