Actually if you are going to prime I believe that you want the mating surfaces primed first vs just the exposed surfaces. I did see some pictures of mating surfaces that were not primed that had hidden corrosion but do not remember where I saw that. The gist of that subject was with two surfaces held together there is more opportunity for moisture to not be able to dry out. hence the possibility of corrosion. On the flip side its all Alclad aluminum is coated.
This is the answer I got from vans aircraft about QBs
The QBs are primed, then assembled. It would not make any sense to do it the other way as there would be no primer between mated parts.
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