The best tool I found for getting the old seam sealer out is a #11 x-acto
knife.  Tried a dremel with wire brush bit, but it just wasn't getting the
job done.  I really second Ed Hinchey's advice about tackling the seam rust
from the inside as well as out!  On the Westies it's worth pulling some of
the cabinetry out to get at the seams behind the sink/stove -- if you can
see rust on the outside there's plenty more back in there and since rust
work is a pain in the arse you might as well do it right the first
time.  Maybe get rid of the old fiberglass batting insulation in there
while you're at it (it holds moisture and contributes to the rust problem),
and do make sure the camper utility inlets are well sealed against the
body.  Apparently water will often leak in from behind these.

I just did a rust attack on my Westy a couple months ago and wrote an
account to the list.  It got archived on the "Been There Done That" website:
http://www.vanagonauts.com/Seam_Rust!171.htm

Good Luck!!

Robert Harris
Ithaca, NY
'84 Westy - Subaru powered
'93 Honda CB750