The finish on the soundboard should be shellac, as this blends with the
gum in the soft spruce wood.  Lacquer only lays on top and does not
protect the wood from drying out.  The new vinyl sealer soaks into the
wood much differently than the old sealers, so this may be a new way to
finish soundboards.

I put on the soundboard finish, thin shellac put on heavy to soak in,
and then sandpaper down hard with 100 grit paper, then put white in
shellac, mop over well so the soundboard is white, then graining brush
it almost dry and about twenty more coats of shellac, let dry then #2
coarse steel wool.

To put on old patina try rubbing glaze into the bare wood (for the
case).  See Professional Refinishing magazine's web site at
http://www.prorefinishing.com/

Wayne Boksa