POR-15 worked well for me - you just have to use it according to the directions and not on smooth metal. If little or no rust is present you need to use some 80 grit sandpaper to create some "tooth" and use their Metal Prep to make sure the POR-15 adheres properly. I did my entire engine compartment with POR-15; inner fenders, firewall, radiator bulkhead, headlight area and the bottom side of the hood, along with the bottom of both new floor pans and suspension parts. The only place it failed me was on a frame rail that I did not scuff up. Other than that it has held up for 7 years now.
But, to the question at hand - yes, by all means, coat the metal with Rust Bullet - 2 coats. The second coat should be applied approx. 3 hours after the first for proper adhesion - make sure to follow the directions supplied by the manufacturer. And make sure that you do as mentioned above - rough up the surface of the metal to create some "tooth" for the Rust Bullet to stick to. None of these treatments will work on clean, smooth metal. They will eventually peel/flake off if the surface is not prepared correctly.