A lot of people use a rust encapsulator with various brands being the favorite. Rust Bullet, POR-15, Zero Rust, Eastwoods, Rust Bullet
Also search for Chassis Paints there are ceramic paints and others made just for frames
there pros and cons to each one I suppose and everyone has their favorite. With the rust encapsualtor your are supposed to seal the rust forever and able to paint over rust (POR?) But most folks go to clean metal anyway
I used POR15 on a good bit of my Pig - inside fenders, floors, etc. It was thick and tough. A friend used RustBullet and it seemed to work about the same (at least in the short run) and was much easier to prep for.
Now that I'm maxed out my Cruiser budget I needed some paint for my frame. I ground away the rust then used multiple coats of Rustoleum rust primer and then multiple coats of Rustoleum gloss black. I know it is not nearly as good as POR15 or RustBullet but it was a heck of a lot cheaper and easy to apply. My Pig is also not a full resto - just a trail truck.
I used the eastwood rust encapsulator followed by their chassis black, it comes in spray cans. I did it last summer and so far so good. I think with any product the final result is going to rely more on your prep than which of several good product lines you have to choose from