Metal birdbath is rusting, badly

I bought a metal birdbath and to keep it from rusting, I sprayed it with Rustoleum. That seemed to make it worse! It has not only rusted, but several layers of ? have been peeling off. I guess it's the Rustoleum and the metal peeling. The birds have discovered it, after a rain, and are loving it, but the water gets nasty. I clean it and put in fresh water, but I need to do something like sand it and put some type of sealer on it. Does anyone know what to do??

Paige