I've used Rust Bullet, Por 15, and Eastwood rust encapsulant and rust converter. Each have their pros and cons. Let me explain and if I get flamed, oh well.
I find rust bullet to be thick (hard to spray) and provides an ugly surface when dry even when thinned. Apply with brush at your own risk. God help you if there is existing rust, it may actually bubble. I would ONLY use it if the area is never to be seen again. In the end, if it rusts, you may never know.
I find Por 15 also to be thick, but not as bad. I don't like the cure time or the final surface. Its prettier than Bullet, but still ugly. Good for wheel wells or rarely seen subframe. I really hate their high-temp stuff, but that is another post...
Eastwood rust converter (the chemical) can convert surface rust well, but for heavy rust, don't bother. I would remove as much physically as possible if you can. When dry it is easily coated. It seems to do the job but can't be left alone and needs a topcoat. I would happily use this on almost anything, but not without a topcoat.
Eastwood rust encapsulator is my go-to rusty metal primer. It is not as thick as others, can be thinned to spray pretty easy, levels well and can be sanded if needed. i use this exclusively for complete undercoats as a primer usually followed by a hard-surface underbody coating. It seems to hold the rust at bay and provide a nice solid layer for the coating to bond too. I have no complaints other than the price at times. If its not rusty, I stick with Epoxy primer.
Bonus: The Eastwood zinc-based frame coating (with the spray tube) works surprisingly well and can really coat interior parts quick and easy. If you do it it can really sneak past welds, so be ready if you have a pretty paintjob already done.
RGAZ