Inside the car, any number of things can cause a mess after you've tidied up. But on the outside surface of a freshly washed, shiny clean car, dirt appears mysteriously. Over a period of days, the paint takes on a dull appearance, windows streak with spots of dirt and any water splashed onto the car makes an even bigger mess. When a clean car feels so slick to the touch, what makes the dirt stick?
When your car's paint was new, chances are the manufacturer applied a top coat, scratch coat, or gloss coat that protected the paint and the metal under the paint. Over time, dirt adhesion, friction from washing and brushing against things such as bushes (for those few SUVs that actually go off-road) will damage this top coat. The scratches left can be microscopic but every one will increase the surface area available for dust, pollution, pollen, bugs and other bits of dirt to cling to.
If you've ever watched dust float in a narrow sunbeam, you know the answer. The air all around us is filled with tiny particles all the time. When a car travels at speed, the air flow over the car creates places on the car's surface where the dirt particles will first adhere. On a medium-sized hatchback, for example, these spots are immediately behind each wheel and along the top of the rear window. Over time, the width of these dirt-collecting zones increases as new dirt clings along the edges of the zone. Dirt sticking to other dirt will be limited by the air flow over the vehicle. In other words, loose dust will blow off.
Shouldn't leaving your car out in the rain wash away dirt? Rainwater seems so pure, so clean. The truth is that microscopic dirt particles can be within rain water. Water molecules form around and bind to dirt particles. As the rain falls to earth it collects pollutants from the air and delivers them to surfaces. When these "dirty" raindrops dry, they leave behind the pollutants they collected. In areas of acid rain, usually areas thick with industry, this can be paint-eating acid salts.
Nanotechnologists have developed substances to apply to car surfaces that will repel dirt adhesion and allow simple water droplets, as from rain, to carry the dirt away. The technology will also work to keep windshields clear from rain without use of the car's windshield wipers. See Resources.
