Go to this page ... Zero Rust
It seems like it's exactly what you want. From Zero-Rust Prep Step Concentrate which will soften the rust for easier removal and then solidify the remaining rust for a tighter surface which will maximize the adhesion of at least two applications of Zero Rust. Once Zero Rust is applied and cured you can then prime and paint the existing surface. I'm selling the stuff but only because I use it widely and am astonished at how reliable it is. Here's the bonus it's relatively cheap for what you get ...