Pure water has a pH of 7.0. However, normal rain is slightly acidic because carbon dioxide (CO2) dissolves into it forming weak carbonic acid, giving the resulting mixture a pH of approximately 5.6 at typical atmospheric concentrations of CO2. But acid rain falling in the United States as of 2000 can have a pH as low as 4.3. At this pH level, rain water becomes corrosive, and can damage many surfaces including automotive finishes.
According to the U.S. Environmental Protection Agency, there have been numerous reports of damage to automotive paints over the past two decades. The reported damage typically occurs on horizontal surfaces and appears as irregularly shaped, permanently etched areas. The damage can be most easily observed on dark colored cars, and appears to occur after evaporation of water beads. Chemical analyses of the damaged areas of exposed test panels indicate elevated levels of sulfate, implicating acid rain. Usually the damage is permanent; once it has occurred, the only solution is to repaint.
According to the United States Geographical Survey, 85% of U.S. households have hard water. The main hardness causing ions are Calcium (Ca2+), Magnesium (Mg2+) & Bicarbonate (HCO3-).These ions or minerals are normally addressed as scale in the water and can cause staining and etching of automotive paint. For this reason, the directions on most car wash products suggest that you thoroughly dry your car immediately after rinsing.
When water that contains acid and mineral contaminants evaporates from your car’s surface, the contaminants are left behind and can stain or permanently damage the surface. This condition is made worse by “surface tension.” Surface tension is the tendency of liquids to reduce their exposed surface to the smallest possible area. A drop of water, for example, tends to assume the shape of a sphere. This phenomenon is attributed to cohesion, the attractive forces acting between the molecules of the liquid. When rain, or tap water from a hose, falls on a car’s surface, the water forms little droplets—or “beads.” Beading of water on the surface of an automobile occurs because water does not adhere to paint, and surface tension draws the water molecules toward the center, preventing the water drops from spreading out over the surface.
Typically, the application of car wax or silicone polish to a car’s surface will increase surface tension, making the contact area of the water beads smaller, and consequently the water beads become taller. This means that the same amount of contaminants will settle to a more concentrated area, and the potential for damage will increase. When the scientific facts are considered, it seems ironic that car owners have come to view water beads as a good thing, believing that they are evidence of a superior finish when, in fact, dried water beads leave unsightly water spots behind that can stain and damage the paint.