Many metals are naturally susceptible to oxidation. This is the reaction of the metal with oxygen to form an oxide. Steel, which is mostly iron, is highly reactive to form iron oxide, also known as rust.
One way to prevent rust is to create an alloy of steel that naturally produces a non-iron oxide (typically chromium) on the surface that prevents oxygen from getting to the iron ions. The other way is to coat the surface with paint or some other type of coating.
For a coating to adhere properly to the surface, the surface needs to have both a slightly rough finish (this increases the surface area for bonding) and free from particulates or contamination (this can lead to delamination of the coating over time).
The part on the left in the picture shows rust and general dirt builds upon the surface of the metal. In this case, the part was tumbled with an abrasive Ceramic Media for only an hour to clean up the surface, remove the rust, and leave microscopic ‘rough’ finish. This is a wet process so it’s important to make sure the parts are dried properly and fully after tumbling to prevent flash rusting.