The best way to rid the area of rust is to remove it . There are lots of converters and oils and over ups , but what you have to do is think of rust as cancer in the human body . Do you want to put oil on it or convert it ? I do not think so .
What I would suggest is to clean the area of any undercoating , till you see no rust . This can be done with a scraper with or without heat gun used it very sparingly . Wash down area with lacquer thinner and Scotchbrite till clean. Spot blasting would be a good idea , but not necessary . Then etch the area with muriatic acid (available at pool supply stores , masonry supply , hardware stores ). that will take you down to clean bare metal . Then you can wash down area with a metal prep wash to neutralize the acid and it puts a temporary rust preventer on that will hold for a few days . Then go back with epoxy primer , and then some kind of top coat paint . I do not suggest going over it with spray can
primer , as that paint is very porous and will let water through .You would be better off with a spray can top coat . You can get any kind of paint in a spray can these days . Just find a auto body paint dealer ( look on line for you area ), that are always more then happy to answer questions and help people out who are learning. Then undercoat that area with spray cat undercoat .
Or you can do as most people do and throw some rust converter on it and spray undercoating on it . your call
