I had a nasty spot on my iaito for years, from a chestnut I cut. I didn’t clean it properly so it was this for 10 years or so. The iaito is aluminium so it never rusted, anyway the stain looked worse and worse. It came of really easy with this fix.
It is also supposed to be very good for polishing away rust from metal. I don’t have anything rusty to clean, so I can not testify this myself. But I do believe it could work if you have a sword that got rusty and stained.