sometimes i think clay is used too often in some cases, or used when not necessary. here is an article from carcareonline i'd like to share.
"There have been tremendous amounts of advertising and "hype" about overspray clays. Let us set the record straight. Overspray clays have been around since the 1930's (No, they are not brand new.) What is new is the marketing of a product for a use other than it was designed.
Overspray clay is a plasticine matrix with an abrasive intermixed throughout. It was designed and is still the best product to abrade (read grind off) fresh paint overspray from existing paint. This is a classic scenario of its designed usage. A body shop repaints a fender of your car and during the preparation process does not mask off the entire car. A little overspray finds its way onto other areas of your car. This overspray consists of tiny little dots of paint on top of the existing paint. The easiest way to remove this unwanted overspray is to "grind" it off with the clay. Copious amounts of lubricant such as Meguiar #34 Final Inspection or One Grand Show Off are sprayed on the paint and the clay is hydroplaned across the surface to abrade the little bumps of fresh paint from the existing paint. The body shop person "floats" the clay back and forth across the paint using only lateral pressure until the resistance ceases, indicating that the unwanted droplets have been ground off.
Many people are touting overspray clay as the new way to "clean" your paint. This is somewhat like washing your face by peeling off the top layer of skin. You will uncover a layer of fresh clean skin, but this may not be the most comfortable nor beneficial experience for your face. Clays smooth the surface of paint by grinding off the microscopic high spots of your paint, much like sandpaper smoothes a piece of wood. They are the best way to remove paint overspray, tree sap build up and other impossible to remove surface contaminants. Realize that you are also removing some paint with the contaminants, so usage of a clay should be utilized only after all other more gentle methods of cleaning your paint have failed.
Overspray clay is a double-edged sword. It can be a scratch waiting to happen. It must be used with only on a well-lubricated area and must be continually checked for contaminants. If you rub it on areas of the paint that have not been well lubricated or a piece of grit lodges in the clay, you have made "sandpaper" that will scratch the paint. The trick to properly using a clay is use copious amounts of lubricant and "float" the clay over the surface. The clay works best when it hydroplanes over the paint surface. You must continually refold the clay to expose a fresh clean surface. If contaminants lodge in the clay, simply tear off the section and discard. Once you have ground off all the surface contaminants, wash with a car wash, dry thoroughly, apply a quality polish to add emollient oils into the paint and finally wax."
But I'm shooting for well before that!