Car wash soap does not remove all the surface defects in your paint; it only gets rid of what can be rinsed away. A clay bar removes bonded contaminants: the stuff you feel when you run your hand across the paint. Things like tree sap and road grime can stick to the paint like glue. Clay bar grabs these contaminants and pulls them off the paint. Start with a clean bar, lightly mist the paint with quick detailer or a mix of water and a little car wash soap (to act as a lubricant) and rub the bar lightly across the paint.