If Your Car is gonna be a Garage Queen POR15 will hold up just fine , and so will the stuff in a rattle can. But if You are going to drive it expecially in the rain or Snow it will NOT hold up.
I spent a considerable amount of Money along with a lot of My time coating the Frame of My then new 2002 Silverado , I followed the directions to the T , used the Marine clean , and Metalready . I even cleaned with Metalready and wire brushed everything again 2 years later and recoated with POR15.
I live in the Rustbelt and this Truck is My dayly driver .It held up fine for about 5 Years then it just started to peal ,now Rust is everywhere POR does not stick to anything but Hands and Cement. The coating is very tough , but once it gets a chip Water and Salt get under it and the Rust will undermine it like Powder coat on cheap lawn furniture. You can grab a loose end and literally pull the stuff off in sheets. The only thing that I would ever use this product on is to waterproof a concrete block wall .
The one product that I have had good luck with is Rust Bullet and I topcoat it with Eastwood Chassis Black as it only comes in silver. Do a google search for Rust Bullet it's amazing stuff , adheres to almost anything and can be topcoated with other paints without having to buy expensive "tie coat" like POR15. I wish that I knew about it when My Truck was new.
Oh and by the way I also topcoated it with POR's topcoat semigloss black. And it still failed.