I've tried several epoxies over the years, and I have to say, the "big boys" just ain't got their crap together!
Southern Polyurethanes has the absolute best epoxy primer I've used. It sprays excellent, adhesion is great, it sands very well, and has exceeded the top brands salt spray tests by some 1000+ hours. It's an impressive product!!!
This will take you to their website and you will find online tech sheets link on the left column.
Metalshaping & Kustom Paint