That's a cool little knife - glad to see it restored.
To keep it from oxidising, you can apply some oil on the blade when you are not using it for a long time. Olive oil will do just fine if you don't have any professional knife-care oil.
Actually, if you feel adventurous, you could etch the blade with some vinegar - that will do two things: it will from the patina which will prevent further oxidation, and it will make the sanmai construction more visible. The process is reversible and it will not damage your blade.
Let me know if you need any help or tips.
I like the vinegar idea, I was noticing that the hamon was not very visable. I applied some flitz wax as an anticorrosive.
Yep, the patina from the etching goes away with usage and fades with age - well, the difference fades. So, a spa treatment could be good for the looks.
Hamon: technically, it's not hamon. Hamon is the visual effect when mono-steel is differentially hardened - so when you dip it in some kind of acid it etches differently.
What you have is a sanmai construction - the principle is the same - harder steel resist the etch more so there's a difference in colour - but it's actually two different steels.
This is hamon:
Edit: wax - that's OK -anything that keeps the blade from direct exposure to the environment.
nice