Hello,
Additional thoughts.
When it comes to fittings (koshirae), if there is even a hint of thought or intention of restoring always ask a restorer before cleaning, even with mild soap. Sometimes the answer will be, yes, it's ok, followed by additional advice, but quite often the answer will be, no, don't to touch it, not even with mild soap.
When in doubt, don't!
Next, back to the issue of caked over swords and never-dull for re-emphasis.
The proper approach especially if you're not absolutely certain of what you have is to send it to a trained professional and have a window opened. There have been a number of times that I've witnessed where a nothing looking sword turned out to be something.
In one case, years ago, a tanto caked over in rust had been passed over by many collectors, why, because it appeared to be a late Koto Mino Seki mumei blade, and it was. Except it turned out to be a Soshu copy made by Hiki Sada. And in fact, what revealed that it was something more was a small window. The point being had it been nevr-dulled, the story might have had a different ending.
When in doubt, don't!
