Maintenance and rules of the Japanese Sword

WD-40 on a katana blade is one of the most common mistakes new owners make. It leaves a solvent residue that evaporates and draws moisture back in, accelerates rust, and permanently contaminates the hamon. We've seen it ruin blades that cost $800. Here's the correct protocol — the one that actually works.

This guide covers everything a new functional katana owner needs: the right tools, storage conditions, how to draw and resheath safely, handle removal and reassembly, proper oiling technique, and what to absolutely avoid. If you've just received your first katana, start here.

What Tools Do You Need to Maintain a Katana?

You need four items. Everything else is optional. Get these before the sword arrives.

Katana maintenance is not complicated, but it requires the right materials. Substituting household products — paper towels, olive oil, metal polish — causes real damage. The tools below are purpose-designed for high-carbon steel and cost under $30 combined.

How Should You Store a Katana to Prevent Rust?

The two biggest storage mistakes are humid environments and horizontal storage without a bag. Both cause rust faster than you'd expect.

Store the katana in its sword bag (shirasaya bag or padded case) when not in use. The bag limits airborne moisture and dust contact. Lay it horizontally — vertical storage puts long-term stress on the saya and can warp the fit over months.

Ideal humidity: 40–60% relative humidity. Below 40%, the wooden saya can dry out and crack. Above 60%, moisture gets trapped between the blade and the saya's interior — the most common cause of invisible rust on stored swords. If you live in a humid climate, a small silica gel packet near the storage area is a $3 fix that saves an $800 blade.

• Never store in a basement or garage where humidity swings dramatically
• Avoid direct sunlight — UV degrades the tsuka wrap and saya lacquer within months
• Keep away from heating vents and air conditioners (extreme dryness cracks lacquer)
• Check the blade monthly — remove from saya, inspect for haze or discoloration, re-oil if needed

How Do You Draw and Resheath a Katana Safely?

Drawing is a two-handed technique with a fixed grip. Improvising it is how people cut themselves or damage the blade.

Hold the saya (scabbard) in your left hand from below, with your thumb resting lightly on the tsuba (guard). Grip the tsuka (handle) in your right hand from above. Keep the edge facing upward — this is not optional. As you draw, the blade's spine (mune) rolls along the saya's interior. This is the correct motion. If you tip the edge into the saya, you'll scratch the blade and damage the wood.

Draw in one continuous motion. Do not stop mid-draw — the edge can catch the saya's koiguchi (mouth) and nick the blade or split the wood. Before drawing, always verify the mekugi (bamboo retaining pin) is fully seated. A loose mekugi means the handle can separate from the blade at the worst moment.

Resheathing (noto):

1. Bring the blade back to the saya's mouth, spine resting lightly on the koiguchi edge.
2. Guide the blade in slowly, maintaining edge-up orientation throughout.
3. Use your left thumb to feel the blade's progress — do not look directly at the blade while sheathing.
4. Seat the habaki (collar) fully into the koiguchi until you feel it lock.

How Do You Remove and Reassemble the Handle (Tsuka)?

Handle removal requires a specific technique. Force it wrong and you split the tsuka or crack the habaki seat.

First, remove the mekugi. Locate the small bamboo pin running through the tsuka — it's visible on both sides. Use your mekugi nuki (brass punch) to drive it out from the smaller end. Set it aside safely; losing the mekugi means the sword is unsafe to use.

Removal technique: Hold the blade at chest height, edge up, with the tsuka pointing down. Form a fist with your free hand. Strike the inside of your wrist on the hand gripping the tsuka — this downward percussion loosens the fit. Repeat 2–3 times. The tsuka and fittings will slide free. Do not yank or twist.

Once separated, you'll have the bare blade with habaki, the tsuba, and one or two seppa (spacer washers). Examine the nakago for rust or loose areas where the mekugi passes through.

Reassembly order — this sequence is fixed:

1. Slide the habaki onto the nakago first — it seats against the blade's shoulder.
2. Thread the tsuba onto the nakago, then the seppa (one on each side of the tsuba if present).
3. Slide the tsuka onto the nakago until fully seated.
4. Tap the heel of the tsuka gently on a padded surface to seat everything firmly.
5. Reinsert the mekugi from the correct side and seat it flush — do not drive it past flush.

How Do You Oil a Katana Blade Correctly?

Two to three drops. Thin film. Never saturate. Excess oil pools inside the saya and stains both blade and wood.

Oil the blade every 3–6 weeks during storage, immediately after any cutting session, and every 2 weeks if your environment exceeds 60% relative humidity. A blade that's been handled without gloves needs oiling the same day — fingerprint acids act fast on high-carbon steel.

The correct oiling process:

1. Remove all fittings (tsuba, seppa, habaki) from the blade to expose the full surface.
2. Place 2–3 drops of Choji oil on a clean nuguigami or lint-free cloth. Do not apply oil directly to the blade.
3. Starting approximately 1 inch above the nakago shoulder, wipe the cloth gently toward the tip in smooth, even strokes. Maintain edge-up orientation with the blade angled away from your body.
4. After reaching the tip, wipe the remaining 1-inch section near the nakago downward — this sequence prevents fiber drag across the edge.
5. With a second clean, dry cloth, remove the excess. You want a uniform thin film — just enough to catch light evenly across the surface. Any pooling or drips means you've used too much.

How Do You Remove Old Oil and Clean the Blade?

Clean before you oil. Old oil that's been sitting for weeks can become slightly acidic and acts as a rust accelerant, not a protectant.

Use a clean, dry nuguigami to remove the previous oil application. Work along the back of the blade (mune) first, then the flat (ji), then lightly along the edge side — always stroking in the direction from habaki toward tip, never back-and-forth. Back-and-forth motion creates micro-abrasion on the polished surface over time and can disturb the hamon if done repeatedly.

For heavier buildup or haze, use the uchiko powder ball. Tap it lightly against the flat of the blade 4–5 times to deposit a thin layer of powder, then wipe with a nuguigami in the same direction (habaki to tip). The powder acts as a very mild abrasive that lifts oil residue and light oxidation without removing steel. Use it 2–3 times maximum per cleaning session — overuse strips the polish.

What Should You Never Do to a Katana?

Most damage to katanas is self-inflicted. These are the specific actions that ruin blades — not neglect, but active mistakes.

• Use WD-40 or machine oil. Both cause rust. WD-40 evaporates and leaves a residue that attracts moisture. Machine oil is too viscous and pools in the saya.
• Touch the blade with bare hands. Fingerprint acids corrode high-carbon steel fast. Handle the blade only with nuguigami or clean cotton gloves.
• Store in a damp environment. Humidity above 60% RH creates rust inside the saya — invisible until the blade is badly damaged.
• Attempt DIY polishing. One pass with sandpaper or kitchen abrasive can scratch the hamon permanently. Don't do it.
• Draw fast or swing at sword shows. Always ask permission before handling someone else's katana. Never draw quickly in company — it reads as a threat and is unsafe in close quarters. Never swing a sword indoors or near people.
• Remove the mekugi mid-session. The mekugi is the only thing keeping the blade attached to the handle under cutting force. Never practice or cut with the mekugi removed.
• Use the blade on hard targets without checking the edge. Test cuts on green bamboo or tatami are fine. Concrete, bone, metal, and hardwood will roll or chip the edge immediately.