Carbon steel is susceptible to surface oxidation (rust) when exposed to moisture, humidity, or skin contact, but it does not rust easily with basic care. The standard prevention method is a thin coat of choji oil (clove-infused mineral oil) applied to the blade every two to three months. In humid climates, monthly oiling is recommended. Avoid touching the blade with bare fingers, as skin oils and salts accelerate oxidation. If minor surface spots do appear, they can be gently removed with uchiko powder (a fine polishing stone powder) followed by re-oiling. The saya also helps protect the blade by providing a sealed environment when the sword is not on display. With this routine, carbon steel samurai swords remain in excellent condition for decades.