if it's pretty localized, you can grind it until it's bright, dimple it in slightly and use JB weld. If the panel is pretty much rotten you need to weld in a patch.
X2. I've used lead in place of the JB weld. Use POR 15 as a top coat, then prime and paint. Prep is most important. If the metal isn't shiny, the rust is coming back.
