Mechanical decontamination is the process of removing bonded surfaces contaminants by means of mechanical abrasion. We scour the surface with a mildly abrasive media. The goal is to use a media and scouring action that is aggressive enough to break the bond, but gentle enough not to mar the finish.
Thing start to get complicated quickly since the contaminants that are picked up by the media are then scouring the surface and may leave marring behind. Detailing clay is malleable enough to absorb these contaminants and minimize the risk of scouring. It's important to kneed the clay regularly to rejuvenate a clean surface on the clay. In this manner, we are able to minimize the potential for marring.
In recent years a number of different clay alternatives have been introduced. These consist of a polymerized rubber applied to some type of media. These vary from blocks, towels, wash mitts and even pads that can be used on a polisher. These rubber products have the advantage of being much faster to use. This is in part due to them being larger and more aggressive. The aggressive nature of them increases the likelihood of marring the paint. It is generally recommended to only use these products if you plan to polish the paint afterward. They also have the advantage of being cleanable. If you drop a clay bar on the ground it should be thrown away, you will never get all the dust and dirt out. If you drop a clay alternative on the ground, it can be thoroughly washed, scrubbed and returned to use. These alternatives last much longer as well, one can easily use it on 20 vehicles and many, many more in some cases, whereas a claybar may only be good or 2 or 3 vehicles.
When planning to mechanically decontaminate a vehicles, it should be thoroughly washed to remove any normal soiling. Then evaluate the surfaces for and feel for any roughness. This roughness is typiclaly bonded surface contaminants that can be removed with clay. Do not bother drying a car before claying, you will be rewetting and drying again in the claying process so drying after a wash is a wasted step unless there is a concern of water drying and leaving waterspots.
The first step is to apply some type of lubricant to the paint. This can be a dedicated clay lubricant, detail spray or other spray applied product. This lubricant acts to minimize the potential for marring, facilitate a smooth movement of the clay media and to suspend particulate that has been removed from the paint. Ideally, enough lubricant should be applied such that it has started to runoff the paint.
The when claying the car, define a work area that is consistent with the media, level of contamination and your skill level. If the paint is terrible and you have a tiny clay bar, work a 1'x1' area. If the car is in very good condition, you can increase that area. Check the clay media after each section and kneed or clean as necessary. Certain areas will be much more contaminated and should be done last if possible; such areas are the rear of the car, behind the wheels and the front bumper.
Once contaminants are remove, wipe off the lubricant before contaminants have an opportunity to settle back onto the paint.
When finished, the paint should feel smooth and be ready for paint correction or application of your protectant.