Understanding the chemical etching process
The emissions from industrial plants and automobiles release sulfur dioxide and oxides of nitrogen into the atmosphere. As they spread through the air, they are picked up by moisture and fall back to the ground in the form of rain drops, mist, fog or dew. By the time these drops land, they have become diluted solutions of water and sulfuric or nitric acid, commonly called acid rain. Because they are diluted they do not immediately etch the vehicle’s paint. However, other factors boost the acidity of the drops to the point where they will etch the paint everywhere an acid rain droplet lands.
For example, once exposed to bright sunlight, the water droplet begins to evaporate. As the water vaporizes, the acid content in the remaining solution becomes much higher. If the day is hot and the sunlight strong, the rising temperature of the body panel beneath the paint can dramatically increase the effect of the acid. The darker the paint, the more sunlight it absorbs, and the hotter the body panel will be. Depending upon the geographic area, the strength of the sun, and temperature, acid rain paint etching may range from a few minor spots that can barely be seen, to numerous shallow craters you can actually feel.