Installed an inner door bottom and lower skin on the left door of my son's 66 GMC. Took the door down to bare metal and now do I prime and then use filler or use filler on the bare metal and then prime?

PPG Omni AU epoxy primer and "Plus 1" fiberglass reinforced waterproof filler.

Dave
66 GMC- father /son project with the "GMC KID"
36 Chevy PU- 4x4 lifetime project
64 Skylark- wife says its hers