Cast Iron Rust Prevention?

Fill the bath with phosphoric acid?

(clip) Is there a rust prevention product I could use in this case...one that is clear so as not to change to appearance of the bird bath? (clip) ^^^^^^^^^^^^ I don't know about clear, but this is what I suggest. Line the bowl with fibreglass/resin, like you would use on a boat. You might add some lampblack to the resin so it does not look like a fibreglass lining. I would prime the rest with a good rust resistant primer, and paint it black. A flat or satin finish would probably look more natural on cast iron than gloss.