I was looking for Hammerite and discovered a new (to me anyway) product from Rustoleum called
Rust Reformer. It's only available in black and it isn't intended for anything other than stabilizing rusty surfaces. While our painted cast-iron bathtub is out for a bathroom rennovation I tried it on several of the rusty surfaces, particularly on the base - the tub itself doesn't have a lot of rust bt the base had plenty! Despite being black it paints over quite well - the tub base is John Deere green and you can't tell through the paint where the black ends and the white begins. We'll see how it works out.
Here's a "before" shot of the tub base. I'll post the finished base when I get a chance.
Joseph