Carbon-steel blades - whether 1045 or T10 - are susceptible to surface oxidation if left untreated, even indoors. The standard maintenance practice is to apply a thin coat of choji oil (a traditional camellia-based oil) or a modern equivalent like mineral oil to the blade surface every few months, or more frequently in humid climates. Use a soft, lint-free cloth and wipe in the direction of the edge, not across it. For storage, a horizontal katana stand keeps the blade supported without stressing the saya's mouth or tip. Avoid sealed display cases without any air circulation, as trapped humidity accelerates rust more than open air does. The lacquered saya should be wiped clean with a dry cloth only - solvents can damage the finish. If you notice any small rust spots, a rust eraser or fine uchiko powder applied gently will remove surface oxidation without scratching the polish.