I've been giving some thought to how piano finishes were applied 100
years ago and in particular shellac finishes (as opposed to varnish
finishes) such as the dark brown mission finish on my piano.

It occurs to me that speed and efficiency were important in factory
production.  For most piano production a hand applied shellac French
polish finish was not practical in a factory setting except on special
high-end pianos.

In looking at the water damaged areas on the lid of my piano you can
see that most of the brown color is also gone down to the wood,  so
that leads me to believe that the shellac was tinted the dark brown
with aniline dye rather than a separate treatment of the wood with just
aniline dye thus saving one step in the process.  The wood would first
have the grain filled with the dark brown filler, sanded smooth and then
one or more coats of the dyed shellac sprayed on with some light sanding
between coats.  After drying it would probably have gotten a top coat of
(possibly) tinted wax.

I just need to identify the shade of brown used on my piano.  I suspect
there was some industry standard back then as to what constituted
a mission oak finish.  More research to do on that,  but I know there
are finishing/re-finishing experts out there who know about what was
used back then.

I have also observed that the surface of these old shellac finishes
did degrade somewhat from oxidation, from sunlight and pollutants in
the air.  If the original shellac coating is thick enough,  they can
probably be cleaned up by a very light sanding and then a re-coat with
a light coat of shellac or just the wax.   I have noticed there is a
slight roughness to the existing finish on my piano, which I think is
just the natural aging of the finish.  Also,  the old furniture waxes 
were made up with some petroleum solvent mixed in with the waxes to form
the paste.  I could be wrong here,  but I'm wondering if the wax coating
might actually evaporate off the surface of the wood over many decades.

It's really only the lid that needs redoing because of the extent of the
water damage,  I think the rest of the piano can have the finish cleaned
and then re-waxed with a brown tinted was.  I can see in the corners
a buildup of dark brown which is not dirt so I think that's original
tinted wax that was left behind.  Perhaps purposefully to enhance an old
patina look.

Dave Krall