To my perhaps faulty knowledge, they didn't realy use a primer like you would paint, say, a car before finish coating. I believe the camo colours were a single lacquer coat applied by spray straight onto the bare metal, forming the protective coat.
Corrosion protection for non-camouflaged areas like wheel wells were most often done in RLM 02 , a light gray/green colour, but exceptions exist for cockpits which were painted a darker gray, RLM 66 in early 1942.
The answer also depends on the time period you're considering. Toward the end of the war, aircraft started to be delivered with large areas, like undersides, unpainted.