Cast iron rusts unless it has a finish to protect it (and even then the finish requires maintenance).
Two coats of any decent metal paint (Rustoleum, etc,) over a coat of primer should limit repainting.
Getting paint to stay under water is even harder.
Usually epoxy type paints last the longest, but even those eventually allow enough moisture through to create rust and damage the finish film.

In the event anyone else with the same problem finds this thread, I want to post the remedy: POR-15 as recommended by the nursery where I purchased the fountain. It supposedly restores the original finish/color and lasts for a couple of years.

Here is a link that might be useful: POR-15

i have the same problem sandblasted twice prime paint ect.... still peeled and rusted not sure what to do might buy a concrete one help

I am going to try rustoleum paint from Home Depot to keep my cast iron fountain from rusting. It's an old cow waterer, and when I moved into my home the fountain basin was filled with transmission fluid and motor oil. I have cleaned all that out and am about to finally paint it and then I will put it all back together. I have ordered a new fountain pump. I will post new pictures and updates about how the rustoleum paint works to prevent rust.

I'm wondering if truck bed liner would be a good option for a base coat on a cast-iron fountain?