Let me start out saying I believed it in a lot. I personally have used several gallons of the stuff before.

I put some on both sides a pair of sandblasted box sides that spent the winter in my heated, dry garage. Both box sides had rust showing through the POR 15 the next spring. The rust was under the POR covering and it wasn't flaking off or anything. I have bare sheet metal in my garage that didn't have that much rust on it after being in the same garage for more then 10 years.

I've had some on the bottom of my 77 Dodge truck cab the peeled off in sheets. I've had some that was still stuck to the metal so well it couldn't scratch off but the metal under the paint was rotted away. Then there is the back of the sheet metal sign in my yard. I spread some POR on the back side of it that was left over from something I was covering with it. The sign was unprepared, I simply POR on the sign back. 3-4 years later, the black POR was starting to fade, so I had some paint left over from painting a truck and emptied the spray gun on the sign back (didn't even wipe the dirt off). 10 years latter, the back of that sign looks great!

The deal with the box sides was the final straw for POR for me. When it couldn't even protect sandblasted sheet metal inside my shop for 3 months, I quit using it. Gene