There is only one way of stopping rust completely for ever... Has
anyone ever heard how they prevent ships from rusting? Well On the
modern ones they use a chemical process called galvanic induction(or
galvanic oxidation, I can't remember :D). This means that on the
inside of the keel they bolt on some pieces of zinc which rust instead
of the iron the ship is made of (I've been told this about by my
brother who studies mechanical engineering at oxford university). The
blocks of zinc must touch bare iron, not painted surfaces. My brother
claims you could do this on a mini and it would require two blocks of
zinc for the front (bonnet exluded), to be bolted to the inside of the
wheel arches, one in each door, two on the inside of each rear wheel
arch and if you want you could also rust-proof the boot lid with
another block. How big the blocks need to be I really don't know, but
I'm sure you can find some useful information on the internet or at
your local library. Anyway you must keep an eye on the zinc blocks
because after they've rusted to a certain extent they need to be
replaced. I don't know if this has ever been done on a car but it
could be interesting to try out!
Greetings

Dantiri