For smaller areas or light rust, using a “soft” abrasive—such as baking soda will take care of any concerns.
Make a paste with baking soda and water and gently scrub the surface along the grain or surface direction with a plastic wash pad or dry cloth until the rust disappears. Once you're done, wash the surface with lots of warm water and let it dry. Within one or two days, the passivated layer should recover and provide excellent corrosion resistance again.
For larger areas, you can apply the baking soda directly to the metal and then mist the surface with water until damp enough to scrub. Again, use a plastic scouring pad or dry cloth to avoid free iron contamination.
If rust is more severe, a cleaner containing oxalic acid might help.
In most cases, it is recommended to try the above treatments first before resorting to abrasive measures.
For extreme cases of rust—or for removing rust from intricate parts or complex shapes—electrolysis is the best approach.
While the exact steps will vary based on the size of your steel parts and tools available, common components of electrolysis stations include:
A current source
A non-conductive pool
A well-ventilated area
An electrolyte solution
The object you wish to clean
A piece of “sacrificial” ferrous metal
The rusty part and waste iron are immersed in the electrolyte pool, and the negative pole of the current source is connected to the rusty part, and the positive pole is connected to the waste iron. When you apply an electric current, the iron from the rust is transferred to the scrap, leaving a black coating on the previously rusted part. It can then be brushed off and the surface passivated.