Hi guys, here's my ten cents worth! Moisture + oxygen = rust. I am 65 and even I have a few rusty bits!
Dry the area out as thoroughly as possible, even use a hair dryer or heat gun. descale the rusted area, wire brush etc. make sure you vacuum up any hard to see, left over scrapings and don't use steel wool EVER, it always leaves a near impossible to see residue. Then treat area with Penetrene first, then Penetrol, then prime with an oil based undercoat/primer or even Gal paint and then coat with an oil based enamel. Enamels of this kind definitely keep out moisture far better than water based products. The paint will stick like the proverbial to Penetrol. I am only guessing that the area in question is normally out of sight so a brush will do! (any brushmarks won't matter). I feel confident that if you do the above even if the area even gets wet, not just damp, it will take a couple of years for anything bad to happen again. But as said in an earlier post by a helpful member, if you can, weld in a new patch and then follow the same finishing procedures. Particularly if you suspect the area to be structural in any way.
And last but not least, and I am sure you are doing this anyway, find out where the waters coming from, Phil

I've been following up my welding with a zinc primer and smothering everything with por15. I learned a lot but after months of fabricating and welding I never want to have to do it again. Interested in the electronic units at the moment and am doing a bit of homework on them. Theres a few types with multiple electrodes. Any ideas which are the better units? How many electrodes for an Escort?? and where do they need to be mounted???
Cheers Drew............

Hey drewdog, sounds like you are doing everything possible really to make sure your beautifully arranged collection of nuts, bolts and sheet metal are given every chance of survival. Well done! 
Electronic rust preventitive units? Well I think if they are as good as they claim, everybody would have one on an old car. But they don't. So is that telling us something? Modern cars are so well made now that we seldom see rust of any kind unless there has been damage to the coatings and not repaired correctly. For example I have a Porsche (sorry!) which came from the factory with a 10 year corrosion guarantee. Galvanised panels are the answer. I have a mate who beach fishes all the time and he has electronic protection fitted, no rust, BUT he also washes/hoses cleans the car after every drive on the beach (old Suzuki 4wd). So does it work? Not really sure...
 
ps I also have a 65 T Bird, 73 mustang convertible, 66 Mustang coupe and 82 Corvette
I am waiting on an answer right now on an offer I have put in on a totally original one owner 1600 Capri Mark 1!Rare as...I think?