The clay bar is a crucial element in the cleaning, correcting, and protecting process of car detailing. More specifically, clay barring a vehicle fits into the later stages of the cleaning process, right before the paint correction stage. After a vehicle has been thoroughly washed and all dirt has been removed from the paint, a clay bar will remove the embedded dirt and other materials stuck in the clearcoat of your vehicle. Driving down the road or even just sitting in a parking lot, a vehicle is always exposed to extremely small particles of dirt that are trapped in the air. Over time these dirt molecules collect on the paint and penetrate the clearcoat of your vehicle, which cannot be washed off through regular wash methods.
How do I know if my car needs a clay barring treatment? Modern vehicles have exterior body panels that consist of a primer coat, paint/color coat, and then a clear coat. The clearcoat is a layer of paint with no pigment added to it, hence why it has no color. The clearcoat is designed to prevent UV damage of the paint coat from the sun. A properly maintained vehicle should have absolutely smooth paint surfaces that are free of bumpiness and roughness. If you skim your finger gently on a clean section of paint, you will notice that the surface is rough or at least bumpy. This roughness and bumpiness is contamination. A clay bar helps to remove this. As a rule of thumb the heaviest amount of paint contamination can be found on the upwards-facing areas of the vehicle including the hood, roof, and trunk, as well as the rear bumper.
A clay barring of the vehicle is especially important to perform prior to polishing because it ensures that the polishing process does not expose dirt particles to the polishing pad. When dirt particles stuck in the clearcoat get trapped in the polishing pad, it can make the polishing pad dirty. And to ensure a swirl-free finish, all materials that are touching the paint surface should be dirt-free.
Furthermore, heavily contaminated paint surfaces literally have a layer of dirt covering the paint surface, which means significantly reduced clarity, depth of gloss, and metallic shimmer (on metallic-colored vehicles) of the paint itself.
Interestingly, a heavily contaminated paint surface can actually hide the significant defects of a painted surface, giving you the impression that the paint is in great condition when it really isn't. Contaminated dirt can be so severe that it will actually fill all of the fine scratches and swirls in the paint. Exteriors in this condition have usually never been waxed in years and have never been washed in many weeks, if not months.
So once the vehicle has been thoroughly washed to remove the initial layer of dirt, and then clay barred to remove the dirt imbedded in the clearcoat, the surface is now as clean as can be and is now ready for the ultimate shine, which can only be produced through car paint polishing.