Don't be fooled by the info that comes with the POR-15. You can't "Paint Over Rust" like they say. You have to buy at least three products to have any hope of it staying on for very long. You need the "Prep step" or some equivalent cleaner, the metal-ready (acid with zinc in it) and the POR-15. Before you even get to the products you have to clean off ALL dirt, rust, etc. Sandblasting is the best, but may not be practical.
Also, you have to completely cover stuff with it. Rust creeps underneath it from any edge that is open. It's basically a plastic (urethane) coating. I have pulled pieces off whole with my hand a couple of years later. Other things that were completely encapsulated have help up pretty well.