Good question. As you've probably already thought about, I guess it depends on the thickness of the por15, (which you already know) and if the por15 sticks as well or better than the bonding agent to the metal and if the bonding agent sticks to the por15, but as you stated, you could scratch up the surface.
If you do the por15, let me know and I can send you some money as I have to por15 the bottoms of the doors on my truck, if you're alright with me borrowing a door bottom's worth of por15.