Run your hand over your car's paint immediately after washing it. Is it rough or smooth as glass? If it's not smooth as glass, you would benefit from a clay bar treatment. It will improve the look and smoothness of the paint. Instead of coating over impurities stuck in the paint like wax does, a clay bar actually pulls them out. No amount of washing will clean these particles because they are stuck in the paint. If you have ever waxed your car by hand, you will notice that afterwards, the towels and pads have lots of black dirt even though you just washed the car. Modern waxes contain slight amounts of polish and managed to pull out some of the dirt that you just waxed and sealed over. For a daily driven car, I recommend using a clay bar once every year or two. If you just bought a car, a clay bar treatment will really help the paint shine.
After you use a clay bar, I suggest following it up with a porter cable polisher and polishing the paint, since clay only pulls out dirt - it does not remove scratches. Then wax your car because the clay removed the wax.
First, thoroughly wash the entire car. Refer to the article: washing and waxing your car for more tips on car washing . As always, stay out of direct sunlight. There's no need to dry the car because you want it wet for the clay bar.
Depending on the clay bar you have, it may come with a spray bottle of lubricant. You can also use a fresh bucket of clean soap and water. It is very important not to be stingy with the lubricant or water because it lets the clay glide over the paint and pull out any contaminants. If you don't wash the car before you clay or don't use enough lubricant, it will scratch your paint, so use plenty of lubricant! Never use just water as a lubricant.
Move the clay in a back and forth motion over a small section section. When you begin, you may feel a scratching, bumpy feeling - this is normal. Keep it wet when moving over a new section. After a few swipes, the passes will smooth out and will start to have no resistance - this means that you have removed all the contaminants from the paint! Feel the paint with your hand and it should be as smooth as glass. Before moving onto the next section, wipe the surface of the clay to remove any big dirt particles and then fold the clay in half to expose a fresh cleaning surface. I suggest starting at the roof and moving down, claying the section behind the wheels last, as this is the dirtiest section.
I recommend using clay only by hand because it lets you feel and control the application. Any machine could increase the likelihood of scratches and you won't know when the paint has been sufficiently cleaned due to lack of feel. This whole thing is pretty self explanatory, but here is a picture anyways. Just keep the surface very wet, move in the direction of the arrows, and you will be fine.
After finishing each small area, look at the clay's surface. If it looks dirty (which it will) or has large particles stuck in it, pick them out and fold the clay to get a fresh cleaning surface and continue. You can also feel how bumpy the gliding is - lots of bumps mean lots of contaminants. If it feels really rough, clean and fold the clay more often to keep the cleaning surface fresh. If you drop the clay onto the ground, throw it out or cut off the entire outside surface. The clay will have tiny bits of dirt and sand too small to notice stuck to it and it will scratch the paint. Because of this, I normally cut the clay bar in half to extend each half's life.
After you're done, you can wash and dry the car again or polish the paint as described in the polishing paint article, depending on how much clay or dirt is on it.
But wait there's more! Save your old clay and you can use it to remove the flecks of dried stuff on the wheels or rock guard. It could be road tar or baked on brake dust. You can use your old clay to clean these surfaces as long as the clay does not contain large bits of sand, etc.
What brand of clay do you find produces the best results? Share your knowledge in the forums here: myturbodiesel.com forums