Por-15 works really good, the key thou is the prep it has to be near perfect for Por-15 to work its magic. I would probably have the frame sandblasted then wipe it down with lacquer thinner or use their product Marine clean, then hosing it off with water letting it dry then using their Metal Ready before you apply the Por-15. If you can get the metal so clean that when you wipe your hand across it no dirt or oil is on your hand then you know you are ready. Also for the Por-15 to work at maximum effectiveness you have to get the frame down to all bare metal.
I ended up having my sub-frame sandblasted and powder coated:
I am very happy with the powder coating, thou I used Por-15 on my block and firewall. I don't have a current pic of the firewall but I do the block:
I had a bit a problem on my block around some bolt holes where I had some oil in then because I didn't want the threads to rust and that was one area where there was some problem getting the paint to stick. Once I touched it up it was fine. The really nice thing about Por-15 once you get it done right is you can clean it with a solvent (paint thinner or brake clean) and it won't harm the paint. I have already done that to my block so I know it to be true. It is also very resistant to chipping and marring (case you hit it with a hammer, which I have done

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Do you plan on spraying the Por-15 or applying it via brush? If brush, I would use the sponge brushes as I think those have a smoother finish. Either that or use a high end paint brush to apply it as the cheap brushes leave brush-strokes.