What is the best prep to apply spray-on bedliner. I know with trucks they scuff up the paint, but is that the best way?

I'm having my 66 Pontiac Bonneville acid dipped later this month. When I get it back I have to have my ducks in a row ready to get it coated within a day or two so it doesn't flash rust. The underbelly will be coated with spray on bedliner. What's the best prep for this? Should I hit it right over the bare metal? primer first? Paint and then scuff?