Evap O Rust found in 1 gallon bottles at Harbor Freight or other outlets would be my choice after manually getting out as much rust in the hollow parts as possible. Once the Evap O Rust has done it's job over a couple of days, you pour it out and it is a biodegradeable product. Hang the parts to let them dry out naturally or by using an air compressor or by sticking a hose down in the tubes and blowing air from your mouth (sounds gross but we bike people are resourceful and are our own air compressor when in a pinch!).
Then the suggestion of the POR 15 would be good and let it dry.
Next step (if restoring) would be to prep the frame with primer, sand and paint it.
Lastly you can pour a liquid rust inhibitor in the open frame tubes but if you want to save the outer patina rust look, be careful not to let it get on the frame you want to save as it will inhibit rust in visually seen areas.
Kerosene is also a trick VW painters use after painting the inner cavities of the rear air scoops on a van or other hard to reach parts that can't get prepped easily. They use a spray bottle to get the kerosene back into those spots and it will wick it's way into the tightest joints and will protect the metal from rusting and not hurt the paint
I've even put some of those tiny bags of silica you get with products sent through the mail in the seat tube to keep things dry in enclosed areas.
Just some thoughts.
Mike Cates, CA.
You could try a soft faced hammer or a hammer and wood block. Tap the opening of the bottom bracket, head tube, and seat tube. That would knock loose rust free and then blow it out with a compressor. The only problem with using POR 15 or any other chemical treatment is keeping it contained inside the frame. You just ruined your patina if any of it gets on the outside of the frame.
I do have an air compressor with a blow tip, maybe that could work. I also have some cylindrical wire brushes I got specifically for cleaning out parts of the bike like this. The only concern I have is getting them stuck inside the tube, as they're short and need a socket extender to get deep inside the seat tube. What kind of cleaner do you use, @Drosentreter? I've heard of POR15 from the VW guys, but I don't have any on hand. I might see about getting POR15, but do I need the full kit, or can I get away with just a small can of POR15 paint? I do have some sprayable rust inhibitor I haven't used yet. Would that work? I'm trying to save as much money as I can right now, and if I can avoid spending any, that'd be even