O dear Gods!

It's the primer wars again. Without coming down on one side or another, if you are going to prime you do so before riveting. The pieces that must be primed regardless of what you decide are all non-alclad. You can usually tell the alclad parts because they have blue or (rarely) clear or white plastic covering them. Anything not clad must be protected and anything that will touch a dissimilar metal (steel to aluminum, for example) must be primed.
There are many acceptable primers and priming methods. Most important is good preparation; the materials are somewhat secondary though most people, I think, like the epoxy primers. For my -10, since I'm not priming alclad, I am using SEM in a spray can; I like the self-etching and it has been, in my experience, pretty tough.
As far as your missing part, contact Vans'; they'll get it to you. It's probably not going to be the first thing you need so you'll have it in hand by the time you need it.
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Patrick Kelley - Flagstaff, AZ
RV-6A N156PK - Flying too much to paint
RV-10 14MX(reserved) - Fuselage on gear
http://www.mykitlog.com/flion/
EAA Technical Counselor #5357