I did my rollbar with that stuff, the bar still had a coating on it, so i just used a scotch brite pad to remove that and painted it, on my through the floor subframes i didnt even prep it. Its all about what your doing, even with my minimal prep it still came out nice, that ****z will stick to anything, the supply shop i got it from had a rusted piece of metal that they just painted right over.
what i can tell you is use a brush thats good quality or actually use a paint roller, i used a crappy brush and cause its so thick it was pulling the hairs out of it and getting stuck in the paint. But you cant beat POR15 for durability, ive hit it with my seatbelts and the metal parts of my harness a bunch of times and there is mabye 2 dings in it.