Definition - What does Rain Spotting mean?
Rain spotting refers to the occurrence of a dried mineral deposit on a material's surface due to rainwater. The moisture acts as an electrolyte and contains salts and/or acids that accelerate corrosion on metallic surfaces.
Metal surfaces exhibiting rain spots have often been exposed to environmental elements and harsh weather conditions that allow contact with rainwater or other natural sources of moisture.
Corrosionpedia explains Rain Spotting
Rainwater naturally falls as soft water from clouds; however as it descends it accumulates lime, chalk and other substances that contribute to its increased hardness. Therefore, rainwater contains significantly higher levels of minerals compared to soft water and distilled water. The rain spots are caused by dissolved minerals or salts that remain after the moisture evaporates.
Rain spotting can be prevented by applying epoxy or other coating materials to the material's surface.