Oiling Your Knife
It is important to care for your knife in the same way you would care for any fine gun, machine tool, or vehicle. Carbon steel knives can rust and should be oiled regularly.
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Keep excessive moisture from tarnishing your blade by applying a light coat of oil any time the knife is not in use. It is best to store your knife outside its sheath if it is to be unused for extended periods, as the leather sheath can absorb moisture from the air.
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My favorite oil is Fluid Film. It is a non-toxic, waxy oil derived from lanolin. It is used on car undercoatings and can be found at most auto parts stores.
Restoring a Rusty Knife
Any product which contains phosphoric acid can be an effective rust remover. You can use any hardware-store rust remover spray or gel. Apply a coat, let it react for a few minutes, then thoroughly rinse off, dry and re-oil.
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Another surprising rust remover is Coca-Cola. It also contains quite a high concentration of phosphoric acid, making it very good for your knives, and very bad for your stomach lining.
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More serious rust might require the use of a mild abrasive. You can use very fine steel wool or fine grit sandpaper to remove pitting. If the pitting is very deep, the knife will need to be reground and polished.