One of the Swedish or Norwigen guys talked about that while ago, he's restocking a musket and he used some kind a tar product. Common up there 2 centurys ago I guess.
If you go here http://www.jimkibler.net/blog/ and scroll down to the section on burl, Jim give a very good description of how he does it.I tried boiling down some sumac the other day, trying to get some concentrated tannic acid to experiment with but it didn't seem to work. Anyone ever try to extract tannins from plants with a high concentration of it. Maybe I'll have to buy some