Stopping rust inside body panels?

Yesterday, I pulled the body plug on the rear corner of the driver's quarter panel yesterday and ended up thumping out a good 3/4 cup of chipped-off coating and rust bits. Looking up inside of the plug hole, it's only mild to moderate surface rust, and thumping the quarter panel all along below the trim holes didn't sound like any of it was threatening to break through.

I don't have any rust-through or soft spots on my car aside from the spare tire well, and the area where everything was collecting at the bottom of the panel is still solid. I'm hoping there's some way I can clean out the inside of the panel and re-coat it to keep the rust monster in check since it's still relatively mild considering the age of the car.

I sprayed some naval jelly up in through the body plug hole and into the trim holes and rinsed it out with brake cleaner as a first measure, but I'm wondering what'd be most effective in getting what surface rust is in there out and then resealing the inside of the panel.

So far I've thought of metal pipe cleaners to get the insides as clean as I can without being able to actually see much of it, plus a different kind of rust dissolver (is there a kind better for getting at relatively closed-off cavities like the rear quarter?) followed by fogging in some self-etching primer and then undercoating paint or standard enamel. Has anybody else done any quasi-preventative maintenance like this successfully before?

I haven't done anything inside the body panels, but Eastwood has an anti-rust product for chassis rails that would work. It's not a paint, it's sort of like a wax type product. It has a long tube sprayer that you just lay in the chassis rail and then drag down. I'd imagine you could use it inside the body panel just as well. It has a nice heavy coating too. The stuff is called Internal Frame Coating.

Make a long thin nozzle for your shop vac. Get it down in there and have at it!

There's two universe's on this subject. In my universe qtrs, trunk pan, qtr extensions come off, b/c they are junk, and sometimes I get to media blast the inside areas that never see the light of day! When I do media blast the inside (see my GTX thread) I usually epoxy prime the inside, it will never rust this way. In the other universe is the delicate approach; usually folks hoping their 40+ yr old metal is salvageable when usually it's not.

rust converter....

I'm wondering what'd be most effective in getting what surface rust is in there out and then resealing the inside of the panel.

I've had a lot of success with this product. It is about the consistancy of water and can easily be sprayed into closed areas, will not harm painted surfaces.
It will only react to existing rust, converting it into a carbon and stopping the corrosion from continuing. Works great on surface rust and you can paint over it.

http://store.interstateproducts.com/Rust_Control?gclid=CL60hOH_q6sCFQUCQAodHUO57Q

I used spray truck bed liner to seal the inside of the body panels. It's not as messy as undercoating IMHO. Either way you definitely want to do something to protect your ride.