Most decorative samurai swords sold today are made of inexpensive stainless steel, so rust or discolouration is never a problem. However, a real, usable samurai sword is made of high-carbon steel and is every bit as vulnerable to rusting and pitting as any other sword. Fortunately, there are many methods for removing rust from these beautiful blades, progressing from gentle to rough.
Rub the blade with a wadding polish according to the manufacturer's instructions. Wipe the sword down with a clean cloth afterwards to remove any remaining polish residue. This will mostly work to remove light oxidation, so you will need to use a stronger cleaner if rust remains on the blade afterward.
Soak the blade in lemon juice or vinegar, which may be acidic enough to dissolve the rust, depending on the severity of the problem. Check the sword regularly to make sure that the acid is not beginning to etch the blade, which will give it a dull appearance. After you soak the blade, rub any remaining rust with wadding polish, as the acid may have loosened and weakened the rust without completely removing it. As before, wipe the blade down thoroughly with a clean cloth after trying to clean the rust off in this manner.
Rub the sword gently with steel wool and sword oil to remove thicker rust. Only rub the steel wool parallel to the length of the blade to avoid scratching it, and only scrub on the rusty spots themselves.
Scrub the sword with a soft wire brush to get rid of heavier scaling or pitting. The brush should not be able to scratch the blade itself, but work gently and only scrub the rusted areas to be safe.
Re-polish the rusty parts of the sword with a polishing stone and oil. As with the steel wool, only grind the stone along the length of the blade, not across it. Only polish your sword sparingly in this way; the proper method for polishing samurai swords takes years to learn, and you can inadvertently damage the blade by overdoing it.
Sand any remaining extremely rusted areas with super fine grit sandpaper. Be careful with this method and only use it if absolutely necessary, as the sandpaper will leave scratches on the blade.
Be extremely careful when handling a sharp sword. Even a blade that does not feel razor sharp can easily give you a deep cut if you run your hand along the edge.