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I have used Rust Bullet on 2 frames with excellent results. I think the overall cost is about par with POR-15 but the preparation is much less demanding. You can just wire-brush off loose rust and brush it on, and I have found that it adheres as well to clean bare metal as rusted surfaces. It can be topcated with any kind of paint that you want - I'm not sure that's the case with POR-15. Here are a couple of pics of the frame with Rust Bullet.
I used POR15 on my plow truck... It didn't hold up The sales person told me I cleaned the surface too well and I should have spent more money on primer
You can never have a clean enough surface to paint on
