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If it looks like black spots on the blade, then you got pitting/corrosion
If it is bluish tinted uniform coating then you have patina.
If you google "knife natural patina" you will get the idea...

Depends on the blade.
Ideally I like to get them as close to new-looking as possible.
The reality is that many blades have issues that prevent them from getting to that like-new look.

There is...