Erase clay bar is used to remove surface contamination that can not be removed easily with chemical decontamination products like Toxin and Blackjack. You should be able to feel these contaminants by lightly rubbing your fingers over the surface, if the contamination is particularly heavy, you will be able to hear a sanding like sound. Erase clay bar should only ever be used on the paint surface after you have removed as much contamination with the chemical products first, this massively reduces the potential to inflict damage to your paint.