If painted before they are bolted, the bolt heads may break right thru the paint film during installation. Powder coating is kind of brittle. I'd leave them factory primed and finish paint them after they have been attached.
I'd feel confident just painting over them. They're galvanized and factory primed. They won't rust as long as nothing breaks thru the paint film, like when the bolts are screwed on to them, and other dings. Also, running a quick bead of caulking around any hangers that aren't tight flush against the surface will prevent rust happening to the bolts screw threads (contact side of the hanger plates) , causing rust stains streaking down.