For rust removal you really do need to get rid of all the rusty metal. This
means cutting it out and welding-in (seam-welding, not spotwelding) new
sections in. Then cleaning the metal with solvents to remove any oils,
priming with red-lead (if you can get it) or substitiute, topcoat and a
rustproofer. The rustproofing should be greasy stuff which REMAINS soft, so
that body flex and dents cracking the paint won't allow water in to cause
rust.

A major cause of rust in previously-clean vehicles is an accident. Any good
thump will crack paint/filler in seams, allowing water in. Salted areas are
death. My 88 diesel Skyline pillarless was fine until it destroyed another
car. Mine had only cosmetic damage. Unfortunately this included cracked
seam paint/sealing, and it began to discolor in the seams. I sold it
quickly!

Andrew Grebneff
Dunedin, New Zealand
VW & mollusc nut
1984 VW Caravelle (currently SVX engine; waiting for a Porsche trans)
1985 Mitsubishi Galant Sigma 2.0 (FWD), for sale
1986 CE80 Toyota Corolla 1.8DX diesel (extaxi)
1989 CE96 Toyota Corolla 1.8DX diesel wagon
1989 CT170 Toyota Corona Select 2.0 diesel (taxi)