The original finish on Japanese rifles was Urushi. Found this on another forum...
By Cabinetman:
The Urushi was, in fact, the finish as you've indicated. However, while it's called a "lacquer", it's really a distillation of a tree sap from a family of trees related to the Sumac family. And, not the pretty, edible Sumac, but the one we called "poison Sumac". There have been many cases of guys who sand away the original failed finish only to break out in a terrible rash, even inside their mouths as this stuff powders up easily. So, keep that in mind.
TK