Knives that sit on a drawer or box all shiny and unused will require little care other than being stored in a dry location. Its pretty different when a knife is used often, maybe daily, in contact with humidity and going through the normal wear and tear any tool experiences.
There’s two facts about knives.
1)Knives have to be sharpened on regular basis
2)Knives rust when not protected with a thin layer of oil.
Even if you maintain your knife well, in many cases a bit of rust is unavoidable. Its not a big problem really. A bit of mineral oil and some used sandpaper is all you need to clean it up. I show how its done in this video.
1)Keep your knife razor sharp. A sharp knife is safer to use.
2)WD40 and other tool oils are good for knives but don’t use them on knives that will be used to prepare food. These are toxic and will taint the food you use it on leaving a bad taste.
3)For knives that will be used to prepare food, you can apply a thin layer of mineral oil or petroleum jelly for protection. Even though you don’t want to eat this directly, they are non toxic and wont leave an aftertaste in the knife. Wipe it dry with a tissue paper after applying it to the blade.
4)While not ideal, vegetable oil can be used. For a knife used for eating meat or used for barbeques, knife expert Abel Domenech says the knife can simply be wiped clean with a paper napkin after use, leaving a thin layer of meat fatty. While this isn’t ideal because it can eventually leave stains on the steel, it does provide some protection from rust.
5)Don’t leave a knife stored long term in a leather sheath, especially if wet or in locations of high humidity. Plastic sheaths are preferable, but make sure they are dry too before leaving a knife in it long term.