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...if it were me, and I decided to do something to treat the inside of the seat stays, I'd look for your local equivalent of "rust converter", like this stuff. Then I'd spray it into the tubes using a small plastic straw (like the kind they sell with some spray lubricants). The plastic straw needs to be the right size to fit into your spray nozzle, and that's usually small enough to fit into the brazing relief hole in the seat stays. Youi spray in a lot, then slosh it around to coat the rest of the stay tubing interior. It's some kind of miraculous chemical converter, and it leaves the rusted surface black, more or less sealed off from the air, and paintable (not a concern in your case.) Or buy it in liquid form (not spray), and figure out a way to get it in the stays by injecting it with something.
But I agree this problem is way over sold as a problem.
ABOUT RUSTEX - RUST CONVERTER
RUSTEX is a rust inhibiting coating which converts rusted metal overnight into a rust ******ing, paintable surface. When applied to rusted surfaces, iron oxide (rust) is chemically changed to iron phosphate, an inert, hard substance that turns the metal black. This increases the paint adhesion and durability of the coating.