I'd suggest POR-15. I have used it a few times. My local Car Quest sells it in a pint or quart can. I buy the smaller six pack size from the maker. Wire wheel off the rust as much as possible. Clean the metal. Paint on two coats of POR-15. Then primer, then your color coat. POR-15 usually takes about 2 hours to apply the second coat and 2 hours to apply the primer. It has to be a bit tacky to apply another coat. I don't know how far back the rust is under your cars seal.
Tom