I have spent more of my time in the last 20 years restoring previously
restored player pianos and reproducers than I have untouched.  I love
the untouched instruments much better as they take less time.  I have
probably seen it all when it comes to sealing metal valves to wooden
valve boards.

PVC-E glue is bad.  Too many have tossed out the gaskets and replaced
them with the plastic glue.  This can not only bend the plates when you
have to rebuild it again, but it can pull chunks of wood up and require
gluing it back in and/or sanding the whole board flat again.  Flattening
out the plates is a royal pain.

Lacquer sanding sealer is not a sealer even though its name says it is;
it is just for sealing pores in wood not for sealing airtight valves.
When it is found in the valve chest, it is easy to get off or melt off
with stripper or thinner or just sanding.

Elmer's glue can be a problem but often novice rebuilders did not know
that it does not stick to old shellac or metal very well, so removing
it is not always a problem.

Bathtub caulk or "RV Roof Sealer" is a nightmare.  There are a couple
of different versions; most of it is clear but it comes in colors.
Getting it off is usually not totally possible -- it takes way more
time to remove it as nothing will melt it and it must be dug off by
hand.  Then you must sand the hell out of the valve board to get down
through the top surface of the wood to get down to something that
anything else will stick to.

The worst is when it is simply slathered every exterior surface in
caulk to stop or prevent leakage, or when it is used in place of or on
top of the valve chest gasket.  You have no idea how big a job it is
to undo _that!_  You may also have to rebuild the pouch board from the
damage.

After all my decades in player rebuilding I have still never found
a use for bathtub caulk in my shop.  Just throw it away or go caulk
your bathtub and leave it out of the pneumatic system.

I have never seen any reason to worry about wood swelling and shrinking
against the metal plates and, while shellac is the most flexible finish
coating used, hence its use on soundboards, there is no shellac that is
as flexible as bathtub caulk and never has been.

If a player piano is subjected to enough moisture to make a huge
difference in the size of its valve board, then the rest of the piano
is probably toast as every hot glue joint in the piano case and back
will be coming apart.  I have had a few of those as well and I suggest
you find a different player piano to rebuild in its place.

Shellac's only problem in this use is its age and just like the finish
on the piano case it dries out and cracks.  But I don't mind replacing
it every 50 years because that is a minimum of time it should last
before drying out.  The burned shellac may be slow drying but it worked
for nearly all the original player makers.

So I must disagree with the original poster since most of the original
valves were gasketed and then sealed with burned shellac and it was
certainly not flexible more than about a week.

One thing that I do to prevent wood swelling and shrinkage is once
the valve board is cleaned every surface, except where something like
a gasket will have to glue, gets coated with shellac.  This keeps the
moisture in the board stable and less able to take on humidity and
swell.

Doug L. Bullock
http://thepianoworld.com/