I am a professional gunmaker and engraver and have used true oil as a base filler for years. Among the other things mentioned above let me say that when true oil or most other stock finish oil gets old it does not work the way it is supposed to. Once opened, unless it is no more than a couple of weeks old You should just go buy yourself a new bottle each time you need it.
Normally I can put on a second coat in four hours. After the grain is filled I let it dry for 24 hours and go to a better finish oil, like permalyn or Dembart. True oil dries too fast for for a good top coat application, in my opinion.
A lot of the guys in the American Custom Gunmakers Guild use the same process.
