I use Por 15 when ever I can.
Used the grey one on the frame, floor, rearend, driveline, bellhousing, suspension and so on.
Even used it on the engine with great results.
It fades a bit to yellowish tan on the parts where it gets hot (engine) and gives it an vintage patina look - thats what I like.
The fading on all other parts is minor but notible.
We can`t buy zero rost over here in Germany, so I can`t say anything about it, but Por15 needs a squeeking grease free surface or you can strip it like cheap duct tape - with a finger nail.
So I sandblasted all parts and used a professionell degreaser.
If you do it that way and don`t min a little fade Por 15 is yust great. I swear by it.
Frank