You can’t paint on top of rust. You have to remove the cancer, or else it will spread through the body, and eventually eat the frame. When you cut out the oxide, you fill in the hole with non metallic putty. Once it dries, you sand it down smooth, primer, and paint it. Since paint wears at different rates, the newly painted area will stand out. To blend it properly, you must sand, prime, and repaint the entire car. Unfortunately, the fixed body work will never be as strong as it was new, and the affected area is no longer magnetic.