The one thing to consider is you can easily sell these O2 cylinders, unless you really want a bell. Among other things, I dive commercially and recreationally and a lot of divers are interested in picking up oxygen cylinders for use in diving. The kind you have is not suited for the actual in water use, but as part of a diving first aid kit, it is invaluable. This is because many dive related injuries require raising the bodies’ partial pressure of oxygen and this is done by administering 100% oxygen to the patient. So to get to the point you can get some easy cash for these and in turn put that into more tools for blacksmithing like the rest of us :wacko: .
If you are still interested in making a bell the best way to wash out a cylinder is using a simple green solution. When I clean a cylinder for oxygen use this is what I use. It is the “standard” for cleaning a pressure vessel for high O2 use (above 36% O2). This makes sure any oxygen impregnated materials that could be fuel for a fire is cleaned out.