Rust occurs when metal reacts with oxygen and water. Oxidation causes metal (steel/iron) to become porous. If rust is not removed, oxygen and water continue to react with the ferrous metal. Thus the decomposition process progresses. The functionality of workpieces or their stability can be destroyed by rust. It is therefore essential that rust is completely removed, especially if surfaces are to be coated or different workpieces are joined together by welding.
In addition to chemical processes, rust is removed mechanically, similar to how layers of paint are removed - by tool brushes, flap discs or coarse cleaning fleeces.
After cleaning, the surface can then be further processed.