Also see: StepByStepCleaning
Also called "quick clay" in the UK, Removes contaminates from the surface of your paint, and leaves it smooth to the touch. It prepares a nice clean surface for the rest of the cleaning process. The claybar may remove very minor swirl marks, but basically prepares the surface for more thorough work. Claybars don't do any damage to any paint or clearcoat, they simply remove contaminates.
There are different grades of clay bars, graded by color. Blue clay bars from Clay Magic, are fine grade, they work well on black and dark color cars such as red, green, blue. Red clay bars are more course and should only be used on light color cars such as white, tan, silver. Using the wrong clay bar may cause swirls so be careful.
Most brands are very similar, some come in kits. Good choices are Meguiars Quik Clay or Mothers Clay Bar Kit from AutoZone or PepBoys for around $17. You'll probably need to look around, as most workers seem not to have heard of a claybar. The kit comes with the claybar (good for three MR2s), detailing spray, and some carnauba wax.
Working with the clay bar is really only part of a larger detailing process; you can't just start with the clay bar.
- Spray detail spray on first small section.
- Using a back and forth straight line motion, rub gently with the clay bar until area is smooth. A few passes and you can really feel the paint getting smoother.
- Don't use a circular motion -- if you go in straight lines, you still get tiny lines, but they're all going the same direction and so they aren't nearly as noticible.
- Repeatedly re-moisten area with detail spray as it dries.
- Make it a habit to occasionally knead and reform the bar so that a fresh portion of the bar contacts your car's paint.
- Use small pieces of the clay bar. If you drop the piece on the ground throw it away -- it'll pick up all sorts of particles that will be bad for your paint.
- Check the clay bar frequently for hard particles. When found, pick them off.
- repeat steps 1 and 2 on next section until done.
Also see: http://www.autopia-carcare.com/inf-clay.html, http://www.claymagic.net/Movies/movies.htm (The bottom video is an advertisement but it gives you the idea.)
Source: http://www.mr2oc.com/showthread.php?t=19101, http://www.mr2oc.com/showthread.php?t=119286, http://www.mr2oc.com/showthread.php?t=29088, http://www.mr2oc.com/showthread.php?p=1327931