You can also use Jasco Rust Converter, available from Home Depot - about $15/gallon, and also available in quart bottles at Lowes. This material is actually phosphoric acid, which is not as dangerous as muriatic acid, but still requires proper personal protective equipment and neutralization prior to disposal if you plan to discard any remaining liquid in the container. It can be neutralized sufficiently with baking soda by dropping the pH below 9, but if you have much liquid remaining, it may take a couple of boxes before the pH drops below 9. Follow the surface preparation recommendations as stated above by wire brushing to remove as much rust as possible before applying. The main thing is to remove the "scabs". These are actually called tubercules and underneath each tubercule is an active iron oxide pit. Inside an active pit is a powdery iron oxide. Try to clean the pit until the metal inside the pit is shiny. This removes the active iron oxide, which is causing the pitting, and exposes the inactive iron oxide, which should be treated with the rust converter.