Some of that rust cure stuff. Paint it on and it turns the rust black. You can then paint over the top of it. I would also take the door card off and spray Wax Oil inside the bottom of the door.
The proper way to do it would be to grind out the offending area and repair with metal. Grind back weld, filler then prime and paint. If you were to just grind out and filler it, you would find that the rust would come back again in 6 months time because the filler absorbs the water when you are using wet & dry, thus holding the water against fresh metal causing rust again.
If you do just use filler, make sure its dried out well before painting.
;-)