Wooden bentos want oil, not soap.

The secret to keeping a wooden bento box from cracking and molding is oiling it regularly and drying it thoroughly — soap is actually your enemy.

Here’s the deal: wood is a living material. It swells when wet, shrinks when dry, and cracks when it goes back and forth too fast. So your two jobs are to keep the wood hydrated with food-safe oil and to keep it dry between uses. When you first get the box, wash it once with hot water and wipe it dry immediately. Then rub a thin coat of food-safe mineral oil or camellia oil (the traditional Japanese choice) all over the inside and outside. Let it sit for 10–15 minutes, wipe off the excess, and repeat a couple of times. That’s your seasoning.

After every use, just rinse with hot water and wipe it out with a soft cloth. Don’t soak it, don’t put it in the dishwasher, and don’t scrub it with soap unless there’s something truly gross on there — and even then, use a tiny amount of mild dish soap and rinse fast. Then dry it thoroughly, both inside and out. Mold almost always comes from trapped moisture, so let it air out fully before putting the lid on for storage.

Re-oil it once a month or whenever the wood starts looking pale or feeling dry. It’s the exact same routine you’d use for a wooden cutting board, and it’s cheap insurance.

Treat it like a wooden cutting board: oil it, dry it, and it’ll last decades.