Replacing the mastic is not as bad as it sounds.  I'm in the process of
fixing seam rust right now, and removing the seam sealer is the easy
part, I just scrape it out with a pointed tool.  The new stuff is white,
so you might get by without painting.

In my case, the rust is far worse on the INSIDE of the van, and I had to
remove most of the westy interior to fix it.  The seam that runs
horizontally on the driver's side looked fine when I bought the van a
year and half ago, and didn't look too bad when I went to fix it, but
some sanding revealed that parts of the sheet metal were nothing but rust
with some paint on top.  On the inside, I was able to make holes in the
floor with my finger!

Bill
'84 Westy
Raleigh, NC