I would recomend that the first step would be to spray any and all rusted areas with a produst called "OsPho". I believe that it is either phosphoric acid or muriatic acid and is available at a marine supply or most larger hardware stores. This OsPho will neuralize the rust and prevent it from continuing. Since it is within your rocker panels, insure that you squirt or spray as deep into the opening as possible. Then fill the hole with Kitty Hair type type BONDDO. I prefer to top the pollyester type BONDO with a finish coating of epoxy mud. Using this method I have never had a recurrance of the rust. USE GREAT CARE AS THE ACID AND THE RUST CAUSE TOXIC FUMES !! Apply the OsPho out of doors and not in the garage.