When you paint an exhaust you need to ensure you do 2 things very well. First is scuff the surface to remove the "shine" so the paint has something to stick to (I've had great luck with grey scotch brite pads). Second, CLEAN it very well...wipe everything down with soap and water, let it dry, then go back with rubbing alcohol to give it a final clean.
I would scrap those paints..no need for primer. I used Krylon BBQ paint on my RAM when I did the entire exhaust and it was bulletproof, lasted for a LONG time without any discoloring, chipping, peeling, etc...
Here is how it turned out...