Search 'Removing Rust With Electrolysis'. I've done that with large antique car parts. When 'done' pour acetone or lacquer thinner into the tubes to wash away remaining water, dry using compressed air and repeat a few times. Pour linseed oil (Waxoil or similar) into the frame, plug the holes then rotate the frame to different orientations to coat the insides. When satisfied let the excess coating drain out. Re-clean the outside and prep for paint!