Pagan News and Views Since 1998
If the cauldron is made of a pourous metal such as cast iron, which if it's rusting it most certainly is. An occasional thin coating of mineral oil will not only shine it up but will help to keep it from rusting.
You can also heat it up quite a bit and use bees wax on it. works the same and I use it for my bush knives. Just don't get it so hot that the wax catches fire.
If you don't mind chemicals you can get a car of the spray sealant. It used to be only black or white but they have clear now as well. Not sure if you ever use the cauldron for food stuffs as well or not. If so go the wax or oil route.