Read this page.
www.sho-shin.com/tang.htm "THE NAKAGO SHOULD NEVER BE ALTERED OR CLEANED IN ANY WAY" if i got anything wrong.
Ijust put part of the quote there for reference and to keep it short.
In the case of Hanbon Forge swords, or swords of similar quality from whatever vendor, I DISAGREE for reasons that require a little thinking outside of the box of dogma;
The swords that they are talking about in those articles are either antiques or simply more fine finished better quality newer swords.
In either case, when those swords were first made, however they were made, I'm willing to bet there was no rust on the tang.
In fact if you'll notice in one part of the article it says, in other words, that the tang can be kept CLEAN for many decades.
So they came clean and they're maintained to stay clean.
CLEAN in this case meaning rust free.
It may be out there, but I have never heard anyone say or read article that says you WANT your tang to rust.
I think it has become misunderstood in some people's mind that rust on the tang, as opposed to preventing the rust in the first place, is a good thing.
That is silly on the face of it.
Rust on one's weapon is a bad thing. And I think the thought process now with those many tangs that have rusted is, "Well, let's make the best out of a bad situation and manage it."
And the reason those tangs rust is because they are simply not as well care for as the blade.
We all know that while not arduous, removing the tsuka to clean the tang and put it back together is a minor bother.
A major bother if you do it EVERY time you clean the blade.
So the tang rusts and they come up with all these great reasons as to why it's "good" to have rust on your tang.
But it's not that having the rust there is good, it's that taking it off at THAT point is bad, because you're going to degrade the integrity of the metal further and if there's any kind of markings on the tang, a smith's signature or whatever, you're going destroy it sooner rather than later.
But that is all on antique swords and other swords that are more finely made where the tang has a reasonably fine finish.
The swords from Hanbon Forge (and when I say them I'm talking about everybody like them) are modern production swords.
Modern, recently produced weapons.
As such they are not supposed to come with rust on the tang or anywhere else.
For the most part the way they get the rust on those tangs is from sword blanks that have been pre-stamped in anticipation of orders and they are waiting around, pretty much uncared-for, in some are of the forge to be used.
Sitting around like that rust forms on both tang and blade.
When a given sword blank is finally picked up and going to be processed for somebody's order the topical rust is naturally removed from the blade in the further manufacturing process (heat treating, sharpening polishing and so forth).
But not so the poor tang that has also just been sitting around in the sword blank storage area getting rusty and only used as a handle, by hands or tongs, in the manufacturing process of the blade with no other care consideration given to it... other than is it roughly the proper dimensions to accept a tsuka.
So eventually the weapon gets into your hands or my hands is not the fine forged masterpiece from the tip of the blade to the tip of a tang that some others are.
It's a Chinese production Katana that rusted because the sword blank was uncared for at some point in the manufacturing process.
Actually, uncared for at all points.
What I do with my non-valuable, non-antique, non-historic, not made by the hands of a master smith katana/tang is that I remove that rust from a very rough finished tang, in order to give it the chance at life it should have had in the first place when it was freshly made a few weeks before it got to me.
So, my recommendation is, if you get a HF sword, or a sword of similar quality with a tang that is not finally finished and it has rust on it from the manufacturing process, take it off.
You're not going to damage the sword, or its value, at all.
Even if you ordered one that has the smith's name stamped in the blade you can probably work around it as I don't think they put a fine finish on the blade before they put the smith's name in it. I may be wrong about that.
But if they do do that (fine finish the tang before they stamp the smith's name into it), that process will take off the rust anyway. So there you go.
On the other side of the coin, if you see absolutely no merit in my reasoning when it comes to these modern productions, freshly-made new swords, with the basically neglected tangs, that you are getting and feel that the rust getting a head start on a brand-new sword is a good thing... well, it's your sword.
Below is a link to how some of the tangs looked prior to cleaning. Didn't really think at the time I would be demonstrating the before-and-after look, so the before pictures were basically only for my own reference with no after pictures. You'll just have to believe me when I tell you they all looked MUCH better after...
HF sword tangs -
imgur.com/a/t5gkCi8