On the body and sheet metal, my local auto paint store recommended Mar-hyde one step, it comes in a spray can or bottle that you can paint on, it will convert any trace rust to black primer. On the frame Por 15 is bullet proof as long as you follow their prep instructions to the letter. If there is any oil, dirt or grease it will peel off, but on just rusted metal it forms a hard smooth paint layer. It is also susceptable to uv light, so if your using it in an exposed area to the sun, it needs a paint top coat. As to what Bob said, I agree. The best way to go is to bare metal, treat with a converter and quickly prime and seal. Small pockets of rust are like beggers, once they get a hold of you they don't go away!