Wash your car and look closley at the paint surface. Run your finger across it. If this finish is anything less than pristine, you're going to notice things with your finger. Things on the paint surface that aren't visible but still there, stuck to it, embedded, adhered to the paint in a fashion that keeps these particles from being removed by a typical washing.

What's happened is the sharper, pointier, stickier things in the air have actually jammed themselves into the first layers of paint. This is no longer a surface-only dirt issue that a thorough washing can handle, as a wash mitt or sponge doesn't have the grabbing, grippiness to pull these things out. So, you're left with a clean car that's still covered with the same selection of glass fragments and fibers, rail dust (little metal slivers emitted by the interaction between train wheel and rail), minute chunks of gravel and Earth byproduct. Enter the clay bar.

Paint clay is a very particular grade of fine clay, not too unlike some clays used in modeling. Used by car care professionals for ages and long a back door secret of the car care industry, insiders have brought the clay bar and its usefulness to public attention. Used with a lubricating spray (often a detailing spray) that allows the otherwise super-tacky clay to glide without marring the paint itself, automotive clay can be the tool to step from "just washed" to "ready to polish and wax."