Ron,
I used POR 15 this past spring to paint all of the corroded areas on my engine. My problem was that every time my raw water pump fails and leaks saltwater it causes a lot of corrosion.
Since these darn pumps eventually leak, I wanted to find a way to protect the engine metal so that I didn't have to wire-brush and repaint every time my pump fails.
On recommendation from a motorhead friend who restores cars, I used the POR 15 to paint all of my corroded areas. Of course I wire-brushed everything first and I used the complete POR system- degreaser, metal primer, and the overcoat.
The directions say that if POR15 is not exposed to sunlight, it is not necessary to paint over the POR 15, and you can leave the metal the black color. I actually painted over it with my Volvo engine paint so everything looks good.
In a couple years when my current pump leaks, I will be able to determine if all of the effort was worth it. It can be used on all types of metal- steel, aluminum, bronze,etc., so it is perfect for all the different metals on our engines. As of this fall, the engine looks great.