Why wood deteriorates
Wood takes in moisture from the air: when humidity rises, it swells, and when it falls, it shrinks. The Canadian Conservation Institute (CCI) states that these very changes lead to cracking and splitting. For a training weapon this is worsened by sweat and skin oils that soak into the fibres; these swell and then contract again on drying, which, according to Japanese maker Tozando, can even warp the weapon. Direct sunlight, per CCI, damages the surface gradually and irreversibly: light wood darkens, dark wood fades. On top of that come nicks and dents from strikes on the edge.
After every training session
Wipe the weapon with a dry, soft cloth to remove sweat and dust. If the surface is already treated with oil or lacquer, a damp cloth is enough.
Oil, wax, or lacquer: sources disagree
There is no consensus. Tozando describes four options. Drying oil (linseed, tung, walnut) penetrates the wood and hardens within the fibres, so water can no longer get in, and the wood keeps its natural feel; the drawbacks are a long drying time, yellowing, and flammable rags. It suits jō and weapons that slip in the hand best. Lacquer protects the wood for years, but the surface feels like plastic; on a bokken, Tozando says it improves grip. Mineral oil must be food-grade, never dries, is slippery, and needs frequent renewal. Wax (beeswax or paraffin) applies easily and is cheap, but protects only the surface and needs frequent reapplication; on a bokken its stickiness helps the grip.
Guidelines from an aikido dōjō (Aikido India), by contrast, recommend teak, tung, boiled linseed, or Danish oil, and explicitly advise against wax or lacquer. Japanese swordsmanship practitioner Shanock recommends only boiled linseed oil diluted 1:1 with paint thinner, citrus solvent, or turpentine; he says tung oil hardens too much, lacquer and polyurethane give a brittle surface, and raw linseed oil dries too slowly. Maker Blizniak's Bokken lists both tung and linseed oil, noting tung darkens the wood less while linseed can take several days to dry. CCI, for museum objects, notes that wood does not need to be "fed", that linseed oil darkens wood almost black over time, and that conservators use microcrystalline wax at most once a year; but this advice targets museum pieces, not a weapon under training stress.
How to apply oil
The procedure repeats across sources. Clean the weapon, put a little oil on a soft cloth and rub in a thin layer along the grain over the full length. Let it soak in for at least 15 minutes, topping up dry spots up to twice more per Shanock, wipe off the excess thoroughly, and let the weapon dry at least overnight, per Shanock 24 hours. Then polish it with a clean cloth. The surface should be smooth and not sticky. Do not train with a freshly oiled weapon. On first treatment, Shanock sands the weapon smooth (finishing with 250 grit) and removes the factory lacquer. He recommends renewing it with a thin layer roughly once a month; Blizniak recommends once every few months, or when the wood starts to look dry.
Rags soaked in linseed oil are flammable; an industry forum states that rags soaked in boiled linseed oil can self-ignite, so put them in a sealed bag and throw them away.
Nicks, splinters, and sanding
Nicks from strikes can be sanded down, per Aikido India. Shanock advises lightly sanding the weapon whenever the surface feels rough, then oiling it thinly again.
Storage and transport
Store weapons upright, not lying flat, in a dry, cool, shaded place. Do not leave them in the sun, near a stove, heater, fireplace, or above a warm-air vent, in a car, or in wet grass. CCI does not recommend attics, cellars, or garages because of fluctuating temperature and humidity. Extremes, per Blizniak, lead to warping, cracking, and rot. Do not try to straighten a warped weapon yourself; Aikido India entrusts this only to an experienced specialist.
When to retire a weapon
Wooden training weapons, per Aikido India, are essentially consumables: retire them at the first sign of a crack or split, as with any other visible damage. A bokken with a crack, then, has no place in training. Check regularly for cracks, splits, and splinters.
Common mistakes
Training with a weapon whose oil has not yet dried. Kitchen oil instead of a drying oil. A weapon left in a car or near a heater. A garage or cellar used as storage. A linseed-oil rag thrown loose into the bin. Trying to straighten a warped weapon at home in a vice.
Which weapons this applies to
These procedures apply to hardwood training weapons: bokken, bō, jō, tonfa, the canne and bâton sticks, training tantō, and other wooden replicas used in training.