A minute later we dunked the arm halfway into a saltwater bath. After only an hour of soaking, the bare cast iron had a light coat of rust. But the area where we applied the RPM was completely rust free. In fact, the water just beaded up on the treated area. This picture shows the RPM-protected area on the left, the rusty area in the middle, and the untreated surface of the arm we kept out of the bath.