I found this extensive review of various por 15 type products.
Very long but worth the watch IMHO, here's a link 41:30 for the summary.

 

Interesting video Tim .I used that master coat silver on a lot of the frame and underside of my 72 and top coated it with flat black rustoleum .I head no idea if it was good or not i bought a couple cans at a local car show. Its held up well so far going on 10 ten years

 

I use POR, KBS and Master Series a LOT, for backsides of panels that are welded, or worked, floors, undercarriages,. I do prefer Master Series, All perform well for what I use them for (NEVER on an outside surface, under nice paint) Some have been protecting the metal for 30+ years. I do have a few failures, more due to impossibility of getting the product in between door seams, and such.
That said, he tries to do a good job comparing them, but I do have some issues with his procedures. If you want a more "scientific" comparison, go to You Tube, and look up "Project Farm"s review of corrosion protection. He takes a more precise method to test these products (not all the same, but similar).

 

I watch a lot of Project Farm comparison videos. The host does a good job of relating his testing to real world conditions.

 

What's the deal with POR-15 anyway? Have you seen this stuff people, I mean have you seen it?? I mean, if it's so good why don't they just paint the whole car with POR-15??

 

Yes, I get the joke, but the guy at Master Series, actually DOES say he primers cars with this coating! POR would not be a good coating, as it is not UV stable, and looks pretty ugly. I have used it as a primer on chassis, and under bedliner or undercoat on the underside of floors and trunks.

 

From my understanding, POR-15 offers an exterior paint now (for tractors and such).