Somewhere in my collection of old magazines about the piano business
is a collection of the Tuners Journal.  If memory serves me today
(which is in question everyday!) then Duco was a finish material
introduced around 1927 to the piano trade.  This was the first of the
lacquer products used to finish pianos.  Prior to that time most old
upright players had a varnish finish on the piano case.

The original finish can usually be tested by using lacquer thinner
to soften the finish.  Old lacquer finishes strip with lacquer thinner.
Varnish finishes do not.  I have always believed the old varnish
finishes were much heavier or thicker than the lacquer finishes.  I do
not know for sure about the soundboard but I believe shellac was used,
and denatured alcohol will test for shellac.

While on the subject of finishes, I have seen some really nice rebuilt
pianos with a cheap gold paint sprayed on the plate.  Originally a
powder known as bronzing powder was mixed with a clear finish and
sprayed on these plates.  If you want to use spray paint then put a
clear coat of finish over it to help keep it looking new.

Don Teach

 [ For best acoustic properties, it seems that the piano soundboard
 [ should be finished like violins are finished.  -- Robbie