These bells are made out of steel and will eventually rust no matter what you do. It soes not hurt the bell or its sound to clean it of existing rust and provide a protective coating such as aluminum paint. If you wish to keep the natural steel finish, you will need to coat it frequently with a thin layer of lubricating oil. Any kind of oil that does not rapidly evaporate will do including linseed oil and cooking oil. Bluing will be expensive and will not protect it for long and would only be transiently effective if the pits and bumps were ground smooth. Curing with heat and oil such as one would do for cast iron cookware will weaken the metal and result in a change in the bell tone and could crack the bell.