Contrary to popular belief, POR-15 (a good product) in and of itself, is not a rust killer. The POR-15 system consists of "Marine Clean (a degreaser), Prep & Ready (a rust converter like Rust Mort, Naval jelly etc), and POR-15 (epoxy coating).
You can use Naval Jelly (or Rust Mort) followed by a good primer and paint with about the same results.
If you like a simple, two step process, many of us use a product like Permatex Rust treatment
or Loctite Extend
or the NAPA Autoparts equivalent.
We then use the appropriate top coat.