Hi Dennis - assuming you plan on more 2k primer or another coat of epoxy, a quick scuff with 180 or 220 is all you need. For me at least, this is the fastest way to cut into the harder epoxies out there with the least amount of effort. If you are working on a very detailed area you may want to approach it differently but on big panels the scuffing really isn't that hard. Personally I like using the epoxy as a guide coat. I have stuff in epoxy that is over 10 years old and have no concerns about adhesion when the time comes.
Deve's point about the thin metal is very valid so my vote is to blast first and epoxy prime if you are going to be a while and presuming you aren't replacing half the metal in your truck to begin with.
Good luck
