...Evapo-Rust is harmless to paint. I just did the interior of the top tube of a Gios frame made of SLX tubing in order to see how far the rust had gone into the frame tubing (SLX is only .6 mm at the thinnest section of the top tube). For smaller parts and components, you can put them in a plastic bag with a ziploc closure and put the Evapo-Ruyst liquid in with it, then just lay it down in a bucket somewhere. Turn and move the parts as needed.

Once I had a good view of what was going on, I rinsed it out and ran some Rustoleum metal primer in there to paint the inside of the tube. Without this I'd have been afraid to ride the thing.

Evapo-Rust is temperature sensitive in terms of how quickly it works, so in my case I heated a little in the microwave before pouring it in the top tube through a funnel and some small plastic tubing. You need to block off one end and any brake cable routing openings.

It's about 23 bucks a gallon....you can find the stores in your area that carry it on their website search function.