With the aid of a DA polisher, No. 390 polish is designed to remove below surface contamination such as swirls, haze, marring and fine scratches. No. 415 is designed to remove other below surface contamination like clear coat staining, brake dust, & oxidation (and other contaminants that can commonly be remedied by hand use). A clay bar removes above the surface contaminants such as mineral deposits, embedded debris and tar.