I use many many coats of pure tung oil. Sometimes I start with a couple coats of tung oil mixed with mineral spirits to help with initial penetration. It takes a long time to do, but if done right the finish can penetrate up to 5mm deep. You have to let it dry for a long time between coats. The oil polymerizes as it dries and turns into a plastic-y protective coating. Smells funny but I kind of like it. It has a Hoppe's no. 9 kinda quality to it.
The biggest problem I have run into with my wood handled khuks is actually from keeping them inside all the time. The humidity can get very low in the house with the A/C running all the time. I've had a couple develop cracks from drying out. My frequent users I just leave out in the garage, so they are always acclimated to the higher humidity outside.