A quick first word about wood since there’s an ongoing search for the perfect bokuto. Wood is not a homogenous material. You can talk about steel or PVC plastic as having this or that strength and expect that your piece has that property. The properties of wood are only reliable in a statistical way. Hickory is, on average, heavier and tougher than pine, but find a piece with a crack, a knot, or one that grew very quickly and was dried to death and you may find it weaker than an old growth, green pine stick. Then of course there’s the way you use your weapon, someone with stone hands and a macho way of practicing that swings wood against wood can expect to go through a lot of weapons. Swing a baseball bat at a telephone pole, do you expect it to last? Of course not. The “full contact” folks are not my favourite customers I’m afraid, if they get a good piece from me I’m a great fellow, if they get one that doesn’t last I’m a terrible guy. That’s fine, I can live with that, but I fear for their safety. Full contact should involve kendo bogu and flexible shinai not naked eyes and bare throats when there are stiff and heavy chunks of wood flying around the dojo.
No matter which way you practice (and please, for your own shoulders’ sake, soften up and absorb those hits) you should care for your tools.
If you find your wooden weapons getting a bit grubby and perhaps a bit sticky, it’s probably the finish. Lacquer or varnish is oddly sticky on weapons, and it seems to be prone to picking up even stickier bits of dead skin and dirt. Now that’s my situation, but for some folks who sweat a lot on their hands, a varnished weapon may be slippery.
Sand the grubby stuff and the varnish off (don’t be afraid to sand your weapons folks) and reapply a drying oil like Tung or Boiled Linseed oil (some “tung oil” finishes, such as Minwax, have varnish so check to see you’ve got straight tung).
If you like oiling your weapons you can use a non-drying (non-polymerizing) oil like lemon or walnut or what have you but don’t let your weapon dry out or it will warp. Speaking of warpage, if you have a jo that develops a slight warp, don’t panic, you may have a “summer jo” and it may straighten out again as the humidity changes. Have a couple of those myself… actually warped jo are all I ever have, the good ones get sold out from under my hands.
It’s the warping that prompts the use of varnish which seals the wood better than the drying oils. If you want to keep the varnish, do clean it once in a while with fine steel wool or a plastic kitchen scrubber and lemon oil, that will remove the dirt and skin and leave a bit of lubricant behind. Wax is about as sticky as varnish and isn’t as good a seal.
If you find a dry spot on the wood and it’s not rough you don’t need to sand it, but you do need to get some oil onto that area to seal it. As wood dries it gets more dense but it also gets less flexible so dry spots will eventually become brittle and splinter.
In general: Green wood is very flexible, but light and dentable as the fibres are far apart and soft. As it dries it gets smaller, more dense and more hard but also as mentioned more brittle.
Dry any wood enough and it breaks, which is why you should always bake your breaking boards in the oven before the big demonstration.