In domestic pianos made between the late 1800s through the late
1920s, rough piano plate castings were almost always filled with a
black material (which I've never positively identified).  This was
sanded to a very smooth finish before it was painted gold.

When the finish on an old piano plate deteriorates due to extreme
humidity, the filler separates from the casting, resulting in a
cracked, peeling finish.  I've seen numerous pianos in which a
technician removed the old finish, including the filler, and simply
painted the rough casting with gold paint.  This treatment looks
horrible, even worse than certain new pianos made in the U.S. by
companies that were failing during the 1970s!

Whether or not using modern metal filler is considered to be acceptable
when restoring antique cars, it is necessary in restoring old piano
plates.  This material doesn't make the plate look over-restored or
_better than new_; it just provides an *original* looking base for
properly-applied gold and clear top coats.

If anyone knows exactly what the black filler is on old piano plates,
and where it may be obtained today, I'd like to try it on the next
rusty plate that we restore! Then, when the next careless tuner chips
the paint, there will be an original-looking black nick instead of a
modern-looking gray one!   :-)

Art Reblitz