If I as going to paint a Vanagon today I would use POR15 on the
seams.First, I would gouge the caulk out.I would sand away till bare
metal.At this point the gap between the two pieces of sheet metal is
exposed as best it can be.I would use a spray gun for a paint and blast
Marine clean through the gap all along the seam. As high a pressure as the
gun can take.Then I would blast metal ready the same way.Then I would
blast POR15 clear that I thinned  10 or 15 percent. Right into the seam!!!
Put the nozzel up against the seam and pull the trigger. Goop will drip
and run out the interior side.So what!! Being thinned out to the very max,
it will use capillairy action to get as far as possiable in remote
locations.When it drys,the metal will be encased forever.May not be a
perfect application, but bare metal is best.Painted metal is ok as the
por15 will stick and not come off.It will protect in places, and protect
by the grip it has on the part.Then I would finish with por
patch.Currently I can only dream on..................or go look at
www.por15.com  and dream on.