Nitrocellulose appeared in 1925 on Victor talking machines.  From
what I've been able to learn, the old piano finishes were a thick
"padding varnish" applied with a pad or brush.

The "varnish" was actually a heavy shellac mixture.  It will easily
dissolve today with alcohol, but not lacquer thinner or much else.
It was applied thick to build a finish quickly.  Oil varnishes could
not have been used on a production basis because of the long drying
times between coats.  Also oil varnish is not affected by alcohol.

Basically it was a drying time thing, plus shellac could produce a
beautiful finish with a fairly quick drying time.  Sprayed lacquer
was extremely fast to use, dry and then work the finish compared to
anything available then.  I've been told by those who know that the
nitro lacquer available today is a poor cousin of what was used in
the mid-twenties.  What isn't?  My $.02.

Bruce Mercer