Water and surface protectants
Water is composed of two hydrogen atoms bonded to a single oxygen atom and is referred to as a polar molecule. Like small magnets water molecules like to stick to one another. This is called cohesion. Water molecules also can be attracted to other substances, such as metal or dirt, especially if they have some static charge on them. This is called adhesion. Some substances, including oils, fats, and waxes, are not attracted to water and even repel it. These are called non-polar substances. Everywhere water interacts with another surface, both adhesion and cohesion are factors. When cohesion is more of a factor, water forms spherical droplets. When adhesion is more of a factor, water forms into sheets.
When water falls on an car unprotected by wax or some other sealant, adhesion is almost as strong as cohesion and the water spreads out into sheets. If the surface is not perfectly smooth water can be channeled along ridges and valleys. Dirt on a car can be “charged” and attract water even more. These tiny flaws may not be readily visible, but on such a surface drops of water appear flat, wide, and often uneven. When properly applied to a clean car, waxes and polishes fill in the larger scratches and layer the whole car. The chemical structure of waxes and other protectants prevent water from penetrating to the surface of the car. Since the protectants are hydrophobic (literally “fears water”), adhesion is much less a factor than cohesion so the water is more likely to stick to itself, beading up higher and rounder than on an unprotected car.