theres some stuff you can get that turns rust back into metal, once you have cleaned the worst of it off paint that on and it should never return as long as its painted properly ill see if i can find a link to it
The best thing all round on a car that nice is to get the screen out, cut out all the rust and if possible a extra 5 or 10 mm of the good metal to be sure you got it all and then have new metal welded in, not cheep but the best way IMO