Faying surfaces.
Assuming you are talking internally, then priming complete assemblies would be pretty much a waste of time. The greatest corrosion risk is between the faying surfaces, i.e. between two bits of sheet metal that overlap, such as a rib flange and corresponding skin surface, or between two overlapping skins. Capillary action draws moisture into these areas, which when evaporates, and gradually salt deposits left behind get higher and higher in concentration. The purpose of the primer is to protect these surfaces within the joints. You will see some people only prime along the rivet lines, so they are targeting the high risk corrosion areas, but saving weight by not priming the bulk of the skin. Alternatively you could fay seal or wet rivet the joints using some type of flexible joining compound so that water can't make its way between the surfaces at all. You will do this in the tanks to stop fuel leaking out, but it is also common practice all over seaplanes because of the high salt concentrations in the water. I have fay sealed my belly skins as this will be the area with highest corrosion risk on my aircraft, especially near the rear as it is a tail dragged. I am using sikaflex pro, as it is very flexible, had a long working time, and isn't nearly as toxic as our beloved proseal. If you dig a little on this forum, lots can be found about corrosion and Fay sealing.
If you are talking external priming, then the above information is probably irrelevant.
Tom.