Well, just for comparison, I am going to try something called rustbullet. It looks like it'll work and it cures via the humidity in the air, much like por15. I'm planning to use it as directed then apply a good coat of paint. This stuff doesn't need to be top-coated and is sort of silvery colored. It will also be subjected to nasty winter driving so next summer I sould know how well it works or doesn't. Check it out.

rustbullet.com