I agree...most of applying por-15 is in the prep. They make this stuff called metal-ready that etches the metal and applys a zinc-oxide coating or some shit like that. The best way to do it is to prep the metal good, then apply por-15, however many coats you want. then, when its still tacky, spray paint over it, that'll create a good bond between the rattle can and the por-15. Thats what i did on my front rigid bumper, and everynow and hten when i scrape it on a rock, the rattle can (i used rustoleum hammered finished paint) will come off, but the por-15 stays underneath. You'll get scratches in the por-15, but the stuff is so thick it wont go through it if you've put it on right.
That being said, i've had por-15 peel off in spots because i didnt prep properly. If you dont prep correctly, it will come off in sheets. Thats why the tell you how to do it right. Its best for rust protection if you can put it on bare metal, but i painted the bottom of my doors, my rockers, rock sliders, bumper, and underbody with it. I sanded everything up good with first, even though i didnt get to bare metal. Then i brushed that stuff on (its self-leveling) and sprayed over it when it was tacky. So far its held up great. No scratches in it anywhere except on my bumper and sliders, but no pinstripping from branches or rocks yet. The only problem is heat tends to make it peel. It has a pretty good heat resistance, but if you put it near an exhaust, it'll come up. Dont know if the diff would get that hot. They also make high heat stuff.
So, just prep it right, and por-15 will work great. Spray over it when its tacky, and it'll look even better. good luck