Is it acceptable to prime and block one panel at a time?
Same here. I could see if you're after perfection to assemble and block it across gaps. But first you'd have to perfectly align the metal panel edges at all gaps by metal shaping with hammer and dolly. That would take considerable work. Then after it's all done and the weatherstripping and rubber bumpers are back on, it would be difficult to get the panels to align perfectly again IMO.I did mine in pieces and it turned out fine.