Rust on Birdbath?

Let it dry out then use the wire brush wheels on a hand drill to clean off as much rust as possible. If rust remains in the crevices try something containing oxalic acid, such as Zud Household Cleanser or Bar Keeper's Friend cleanser, and make a paste out of it and rub it into the crevices and let it set for awhile. Then clean as best you can, (perhaps with a Dremel) rinse well and let it dry out. Ask for suggestions at a big home supply store as to what you can paint on to prevent further rust and will still be non-toxic to birds. Good luck.