Surface rust or body rot?
If it's surface rust, grind it down with a grinder until your see shiny metal. Then use rustoleaum. This primer will "chemically convert the rust into a tough primer which you can paint over".
Rust is cause by the oxidation of metal (IE it is technically combusting at a slow rate). OIl or grease helps to deny the metal Oxygen. Anything you can do to deny oxygen to the surface of the metal is going to help.
Rust is cancer. Cut it out and reweld in new panels.