In the 1980s, damage to automotive coatings from acid rain became a problem. Both dealerships and buyers complained about the ring type or "water spot" etches that were caused by acid catalyzed hydrolysis in areas where acid rain occurred and temperatures were high.
Etches are formed when material is lost from the coating surface. This material is lost as a result of acid-catalyzed hydrolysis of the chemical bonds within the coating. When enough bonds are broken, polymer molecules or fragments become detached from the rest of the coating and are washed away. The lost material is greatest at the edge of water droplets where the acid concentrates during evaporation. This leads to the familiar ring or "water spot" pattern.