Every couple of years I completely strip my westy and
take it in for the Krown rust-prevention treatment.  This
year, under the right side if the rear (fold down) seat/bed I
found rust bubbles under the paint.  When I pressed a bit
on it, my finger went right through into a space right
above a structural beam (that runs under the van).  The
rust hole is about the size (and shape) of an egg.

Also the spot welded seam that runs along under the rear
(hanging) closet is a bit suspicious with rust showing
along the seam.

Also be careful of the horizontal surface just in behind the
tail lights, my left side has a rust hole in it.

I post this for Westy owners who may not check these
places very often.  Malcolm