The POR15 can says to brush it on to sandblasted or rusty metal. I have used it on sandblasted metal and it adheres very well. On my frame I sandblasted, sprayed Eastwood Epoxy Primer, then Chassis Black over that. If the Chassis black chips you can just sand and spray on a little more and it doesn't cost as much as POR15. The epoxy is tough as nails.
Good luck.
