After washing, your car with a normal pre wash and shampoo, you may still feel spots or a roughness to the paintwork; this is stubbornly bonded contamination that requires a clay bar to remove it. After using a clay bar, clay mitt or clay block on the contamination, be it localised or the whole car, the paintwork will feel smooth to touch as if it was glass.
Removing the in ground contamination is a vital step in the detailing process for any step that you may take afterwards, such as machine polishing or applying a protective treatment; it will be hindered by the small spots all over the paintwork. In some extreme cases, the pads used during machine polishing can be ripped apart or the sealants.
Clay mitts or blocks can be used instead, normally less aggressive, so they won't deal with overspray or heavy contamination but are fantastic for light maintenance or speed over an entire car.
Remember, a clay bar can be used on paintwork and glass to achieve the perfectly smooth finish, use only when required as a clay can marr paintwork if used too aggressively. A light polish will correct this marring, but any protective coating may need to be reapplied.
How do I clay bar a car?
Should I use a clay bar on my car paintwork and glass?