I am assuming you live in Canada from your use of kms in describing your friend's vehicle mileage.
I have had a lot of experience with the rust demon, as my father was patchin his cars when I was a young lad. I patched my own cars when my turn came, but farmed out the big jobs to metal fabricators.
The only way to kill rust in existing metal is to remove the rust or the metal in question. It's like cancer -- you have to cut it out, or you can only slow it. This means getting rid of part of the panel, or snadblasting. Get something on there to seal it as fast as possible -- if any moisture is present, it will start on exposed metal very quickly.
Bondo isn't a great answer as it is porous -- it has to be painted to seal it, and keep moisture out.
POR-15 is good stuff, and it adheres very well to metal to stop rust from happening any further --in fact it likes a little bit of rust on the metal to help it grab. For some areas, they actually make a POR putty that can be used to rebuild some smaller problems. But, and this is important, POR-15 must be used correctly in order for it to work. I let some stuff slide the first time I used it, and some rust continued to work.
As for the plastic cladding, it does NOT help. It captures moisture and stops it from draining and evaporating as quickly, thus making cars MORE prone to rust. Ever notice how the rust usually starts at seams or where cladding is held on to the metal by clips and the like? This is why.
If the work is already done, your best bet is an anti rust treatment such as Krown. Otherwise the repairs will be short lived.
Good luck.
