The seams don't rust until the seam sealer cracks.  No cracks, no rust.
 If you see a crack in the seams fill it with something, I don't care if it
is toilet paper.  A crack starts what is called an "oxygen concentration
cell" type of corrosion.  A difference of O2 concentration causes a bigger
voltage difference than most other types of corrosion.  That is why
bridges/pilings show lots of corrosion at the waterline, and little above or
below the waterline. I have never seen a Vanagon with rust at a non cracked
seam.  Just the opposite, I have an 86 Syncro that lived its entire life in
CA.  The whole thing is beautiful.  Except for 1 12 inch section of seam,
which is beginning to rust.  At the cracked seam.
    Now the question, what do you replace the factory seam sealer with?  1
part polyurethane?  Catalyzed seam sealer?  I used very expensive catalyzed
seam-sealer on a repaint of a van and it cracked in 9 months. Anybody got a
good answer to that?  The sealer I used was 30 bucks a tube, wholesale, body
shop stuff.  It wasn't flexible enough.