Lewis,
You are correct about soda blasting removing only light rust. Soda blasters do not recycle the blast media and no cabinet is needed. But, like glass beads, walnut shells and plastic beads are recycled, so a cabinet is required. In my opinion, using either a soda blaster or walnut shells in a blast cabinet to remove rust from an old kettle will be just about futile. You need to do something else, like electrolysis, to get the rust off. About the only thing I use the blast cabinet for is waffle irons and gem pans, and that is after lye, electro, and a good bit of manual brushing.
Jeff