Rust elimination or shall we say controling them comes part of a car's maintenance. A new car's undercoat lasts for just a couple of year so it's wise for a re-application. Minsan kung merong nag-uumpisang kalawang, I just use a still brush then apply rust remover. The following day, you can follow up with red oxide then coat with undercoat spray for the finish. Mabuti yung inaagapan.