When I was an apprentice, we used to bend 2" and 3" thin walled stainless tube filled with ROSIN which I believe is a product of Pine Tar. Once the tubes were bent on a hydraulic tube bender, a steel ball was 'hammered' down the tube by placing a slightly smaller ball or two behind it and shaking them down. Rosin can be found in musical instrument shops for treating violin bow strings. Once heated, it melts and flows like syrup, going hard when cold.
For smaller tubes, we used CERROBEND. A bit of a laugh to saw a teaspoon in half and join the two halves with a bit of cerrobend, and slip them into the mug on the tea trolley. Sit back and wait for the rude remarks !! Life were fun then.