In the UK you can get Black French polish by Liberon.  This seems to
come close to John Phillips' original enquiry.  I have used it and,
whilst you can brush it on, it is very quick drying, so using a cloth
"rubber" usually gives a smoother finish, although getting close to the
almost perfect gloss achieved 100 years ago is difficult.  I wonder
whether they sprayed it on; any ideas?

John Farmer, UK
www.musicanic.com