When I bought my wooden Jian from Tigersden, it came with a very detailed instructions on how to treat them, so that it would increase their lifetime and durability.
In a nutshell there are two main methods that are slightly different, but both should yield good results. Both rely on applying oil to the wood. Teak or linseed oil was suggested by the Graham Cave, but other natural oils should do the trick as well.
Both methods assume you've already taken care to sand off any roughness or splinters from the wood before you start oiling.
Method one — Oil coating:
- Wipe off all the dust and dirt with a piece of clean, dry soft cloth
- Apply thin coat of oil to the weapon and work it into the wood a cloth
- Wipe off the excess oil with paper towel and let the oil soak into the wood
- Wait for a day for the oil to soak into the wood and dry up
Repeat steps 1 – 4 for three to five times (yes, it takes time). The result should be nicely hardened coat of oled wood. This is easy to do and works quite well.
Method two: — Beating in:
If you are doing lots of full contact freeplay, you might want to use this method as it will compact the outer layer of the wood, making it thicker and harder forming a tougher shell. This will increase the durability of your wooden weapon quite a bit.
- Wipe off dust and dirt with a clean cloth
- Apply thin layer of oil to the weapon.
Here you can be a little more generous than with previous method, but see that you will not soak the weapon and make the wood too soft.
- With a wooden mallet, beat the whole surface of the weapon with a light to moderate strength beats. Systematically work the entire surface with the mallet.
If the wood gets too dry, apply some oil. Keep it light though.
You may repeat this procedure few times to make it harder, but let the oil soak in and dry up for a day before doing it again.
In the end you will get nicely hardened weapon that will be able to take full force blows from either other wooden weapons or even a bladed ones without any problems.
There is also a great article on how to oil and break in a new cricket bat, that describes the similar process.