So I'm currently in the restoration phase of my first bug, my '78 Super convertible. After a lot of research before I found this forum on mostly non-beetle specific forums, my dad and I settled on what seems to be a pretty solid method of getting the rust out and long-term protection. It's not really complicated, just grinding what we can to get to the shiny metal below in the spots that aren't bad, and for the spots that are, grinding what we can, then treating it with phosphoric acid twice (as the method in the acid primer thread states). However, then we grind the residue and rust that it revealed off again, and treat it with a zinc oxide (not zinc chromate) primer we have. Then it's ready to paint. On spots where patches are required, the paint is able to be welded around without compromising it as well.
The reasoning behind removing the acid primer residue is that the high zinc content in the paint, which we specifically chose due to it being a non-epoxy based product, causes the zinc to bond with the metal making and electromechanical bond (I believe that's the term, correct me if I'm wrong) which then stops or slows the progression of rust under it due to the zinc acting as a sacrificial element which will go before the iron in the metal does. The paint is 93% zinc content and rated to withstand 10,000 hours of salt fog exposure. It's the kind of stuff they use to cold-galvanize bridges over salt water.
Anyway, I just wanted to share that to see if anyone had any insight as to the method and if anyone else was using zinc oxide primer on their bugs.
http://www.zrcworldwide.com/p_zerovoc.asp