Obviously, there are many solutions out there for preventing rust before it starts. My concern is how to prevent the spread of rust, once its kicked off. My belt line is starting to bubble in places and I'd like to salvage as much as possible of my rear quarters. I would say the problem is currently not critical, meaning I can probably salvage the quarters, when I have the time to actually deal with the problem.

So, any thoughts or known products/methods I can use to stop the spread of rust once its started?

I would clean the area up as well as possible, using sand paper and/or wire wheels. Get rid of all the visible rust you can. Then use a rust converter , and prime it. You won't be doing anything for inside the panel seam or behind the panels, but its should stop/slow the surface rust.

We used a couple different kinds of converter at the bodyshop. Mostly, we used one in a pump spray bottle, you'd spray down the surface, let it sit and then rinse with water. We did this to an entire early 70's Chevy truck we were redoing. Then primed and did body work on top of the primer.

The other kind of converter was aerosol and turned into a kind of primer that could be painted over. We didn't use this much, and not on large areas. Usually on small parts & stuff, mostly personal projects, not customer cars.

I can't remember the name of either specific brand. I did a quick google and didn't find what we were using.

Almost any parts store will have a product that used to be called "Naval Jelly". It is a thick paint like stuff that you can brush over the rust and it will kill it. Like he said, wire brush the loose stuff away and then use the naval jelly. Wait a day and do your bodywork.

The last time I looked it was called rust converter. I used it on Wilma's frame.