Definition - What does Rust Spotting mean?
Rust spotting refers to the occurrence of dried iron salt deposits on a metal's surface due to corrosion. Moisture containing salts and/or acids on a metal's surface acts as an electrolyte and accelerates corrosion, resulting in the formation of rust spots.
Corrosionpedia explains Rust Spotting
Metal surfaces exhibiting rust spots have often been exposed to environmental elements and harsh weather conditions that allow contact with rainwater or other natural sources of moisture.
Rust spots are caused by dissolved minerals or salts that remain after electrochemical corrosion reactions have occurred. In outdoor environments, hard water contains significantly higher levels of minerals compared to soft water and distilled water. The primary natural source of hard water is rainwater. Rainwater falls as soft water from clouds; however as it descends it accumulates lime, chalk and other substances that contribute to its increased hardness.
Rust spotting can be prevented by applying epoxy or other coating materials to the metal's surface.