India ink, or the Sharpie Oil Based paint pen in the extra fine point. Thats what I use. When dry it will not run or smear when wiping coats of oil over it. You still have to be careful if your going to buff the finish. Get a lot of dry coats over it before you do that. I get em on Ebay for a couple bucks each, in different colors.
My crow calls in the crow call section are signed with it, albeit, kindergarten-like writing.
BTW, the Pigma pens run and smear too.
Mark