METAL HEAT TREATMENT, carbon stopoff paste for carburizing
Actually, a number of firms make carburizing and nitriding stop-off paints, used for selective case hardening. The Duffy company, Avion, and Heatbath all make such products, which commonly have a low-melting material such as tin in them. The tin (or whatever alloy) melts and protects the substrate by acting as a diffusion barrior to the carbon or nitrogen. The paints just flake off when the heat treatment is done, which is an additional advantage over the copper plating alternative.
A search on Google, under "carburizing stop off" gave a bunch of hits, including websites for the three outfits mentioned above.
As an aside, I've learned from experience not to use the nitriding stop offs to repair spots where an electroless nickel plating stop off was chipped. The tin combines with the EN, giving an alloy that resists standard chemical stripping solutions. We had to mechanically remove it, which was irritating...