Rust converter (the phosphoric acid type that makes your hands go blue) is good to spray in behind rust repairs, let it dry off for a few days before spraying any aftermarket rustproofing spray. There are some really sticky brown ones that don't sting, but don't use them in the doors as they block the drain holes. Try and get rust converter into the hidden spots too like the front of the door frames between the window support and the hinge carrier (front of door) as they always rust in here as they were bare metal. I also like to drill a small hole (like 3-4mm) on the sides of the cowl where the doors close so you can get some rust converter and then fishoil into the double skinned area of the cowl (bits on the left and right extremities in front of the windscreen). Fishoil sprayed through one of those little plastic tubes that push into the spray can nozzle works good. Once it has all dried you can put a dob of fibreglass (non permeable) filler into the holes before paint. Go and look at a rusty car in a wrecking yard and see where to drill the holes.