With FIRE!
^It sounds like you shouldn't be on a computer if you type like that.
Honestly it depends on various things. If the rust is deeper than surface rust, then you are more than likely going to have to cut out the affected panel, and weld in a new panel. If it's surface rust with no pitting (pitting indicates that the damage is extremely severe and that you will need to sand the metal away or sand/particle blast the area before filling it) then you can get away with a wire brush and removing the surface rust, cleaning the affected area, and finally filling and priming the area with rust conversion primer.
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