A very long time ago I worked in the body shop of a large motor dealership. Although mostly we'd replace damaged panels/doorskins etc sometimes we'd effect a repair using filler. Like I said in the previous post, if you do it correctly, you should have no problem with the rust reoccurring. Although we never used rust killer I always made a point of it when doing work on my own vehicles & always found it better than not. I remember patching up numerous rust blisters & dings on an old banger I owned which I then ended up keeping for about five years & the rust never came back. A good tip for sprucing your vehicles paintwork up (though this only applies to white paint) is to add a small amount of bleach to the water when you wash the vehicle - a couple of capfuls per bucket) & it will bring the paint up like new.
Back to the bodywork thing. Success lies in the preparation. Take your time (spay not brush) & you will be rewarded with a panel which need no further attention.