Autosol

Autosol is a polishing agent for metal surfaces that is well suited for maintaining knives. Many of the Japanese kitchen knives have a core of steel types such as shirogami and aogami, which oxidize very easily or rust directly if you have been careless.

To remove the rust, the discolouration can be buffed away with a dish towel and a bit of Autosol applied to the oxidized area of the blade.