rust on the sword
QUESTION: Is there any practical impact on the sword if it got rust on the blade or tang? thanks.
On the tang, not really. Generally speaking, you don't oil or maintain the tang as you do the blade - and what happens is that it forms a layer of patina that actually protects against further corrosion (indeed, with Japanese swords, cleaning the rust of the tang is a real no-no - especially with antiques, as it will dramatically damage their value!).
The blade is a different story, rust will cause it to lose its edge and eventually - if left unchecked, can possibly cause pitting or other weaknesses.
For most practical purposes, it is a cosmetic issue - but if left unchecked on the blade, it can become a practical one...
Hope this helps.