rust treatment?

[amalventano at sc.rr.com (Allyn)]

ok, i know POR15 is the stuff to get if you want to paint to seal in a 
surface that has started rusting. the question is, what is the good stuff 
that i can use to treat a surface that is _really_ bad. you know, when it 
looks like its an onion peel (feels like it too). some surfaces under the 
passat i'm afraid to even touch (the major one is a brake line that runs 
across the rear, right by that fuel assembly thing). normally i would just 
wire brush something like this to get the loose stuff off, but i dont want 
to start leaks if i can help it...

thx
Al