Well, I wouldn't disagree with Stan the Paint Man, he's got way more experience than I do.
But, I'm currently restoring my tub and I've been using a product called Rust Mort. It's phosphoric acid based and chemically reacts with the rust, turning into a black ferric phosphate, which then can be painted over. I've used it thru-out my tub. It's a clear liquid that you just paint over the rust and let dry.
Almost instantly, it turns the rusted area black. Then, as it dries, it turns a more grayish sheet metal color. Here's a shot of one of my floor pans right after applying the Rust Mort directly over the surface rust. The second pic is after it dried and a light coat of metal glaze was applied.
I can't say how long it will last, but it sure seems to work as advertised, so far.