What Grenadier said.
Note that you can also use most other finishes as well such as poly &tc. However, the "thinner" the finish, such as the various oils suggested, the more deeply they'll penetrate the wood grain and lock the fibers together.
I've also had decent luck with bee wax and even candle wax. They don't last as long (by far) as any of the oils or finishes but they do smooth out the grain, and make the area slightly more dense and cushions ever so slightly against impacts. When I've used waxes, I also used a heat gun to melt the wax until it flowed like water and then heated the wood so that it soaked into the fibers.
My best results (longest lasting and most durable) have been with polyurethane. (note: some people don't like the "feel" of the wood with poly on it - doesn't "slide" the same way in the hand - give some people blisters) The wax was extremely attractive but I think the most attractive results were from the Tung Oil or the Boiled Linseed. They both add a color to the wood. The wax makes it look darker and slightly "aged" but it buffs out in a way that cannot be matched with the Tung Oil or the Boiled Linseed.
Once, on a white ash Jo, I used Boiled Linseed for a few coats, sanded, wiped with water, then waxed. Looked sweet.
I suspect that you could even use Pledge on bare wood though I haven't tried.
As part of your maintenance process, you really must re-fortify with wood oil to prevent the wood from drying out. After you sand, use wood oil. Lemon oil is great but there is also rosewood oil and some other stuff. Heck, Murphy's Oil Soap, even.
You might also consider what wood you're using for your weapons. A lot of boken &tc. are made with oak, which ain't bad, but there are better. Consider buying a hickory boken. Trust me on this. It'll be a bit heavier but it'll be nigh on indestructible.
Peace favor your sword,
Kirk
