To derust simply use a wire brush, hand held, and brush parallel to the teeth. On a crosscut file you need 2 passes, to get both angles of the teeth, then wash with some kerosene ( paraffin in the rest of the world) to clean the debris out. Then after that use a cloth dampened with light mineral oil to rub off all the remaining rust, and store where it will be dry. A wooden drawer that has bare wood, oiled with furniture oil, works best as a rust preventer, as the volatile oil compounds evaporate over the tools with time. You can buy corrosion protector tabs that are basically an oil soaked sponge that do the same.