What to prime

After perusing the forums for a while it seems the general wisdom behind what to prime is all non-alclad parts and mating surfaces. While the non-alclad parts makes perfect sense because they don't already have a protective coating, why prime the mating surfaces? Since everything is aluminum there is no kind of galvanic anything going on between the parts so what is there to protect the alclad parts from?

Greg,

This topic has certainly been debated endlessly, so you won't likely find any consensus on the issue. Opinions vary all over the map. A lot depends on the climate you live in, where you intend to fly, etc. I carefully primed all of my mating surfaces. Not out of any concern for galvanic corrosion, but for extra protection in case any moisture seeps in between the aluminum parts. I've seen damage to alclad aluminum from water spots, and under the edges of the blue vinyl where it had buckled up and moisture had marked the metal. It's cheap insurance. Just my opinion.

corrosion

I am priming everything with a very thin coat of SW P60G for the same reason. I've seen corrosion form under the blue plastic.
I use Napa for small parts.
It also seems to be a plus for resale. I have no plans to sell but who knows.
I even prime platenuts! Yea, kinda sick. I know.
Bottom line. I read the wars and decided I like the idea even though it's adding hours to the build. I'm kind of OCD though.
Most of GA planes have no primer inside and are fine.
It really is builder choice. Vans manual only directs primer be used on non alclad and steel parts.

Not to get into the whole primer wars thing but to answer the question the OP actually asked... Priming mating surfaces isn't for galvanic corrosion but because those surfaces are a good spot to trap moisture. Also, they are the areas that tend to get the cladding scratched during assembly.

The rest of you may now resume the primer wars.

Greg,
Although you mentioned perusal of VAF for a while, I wanted to make sure you were aware of the considerable amount of posts concerning primer.
It is in the "never ending debate" section.

The following is a link to the section on primer:

Primer debates

Be wary of the "view" settings (at the bottom of the page). Make sure they say "from the beginning" and not "from previous month". That will open pandoras box of conversations on primer.

Cessna

I am not sure but think that I read that all new Cessnas now are primed throughout. NOTE - zinc chromate IS available thru ACS - I know, I bought some here in the most regulated state in the US.

Good luck with whatever you choose.

Thanks for the replies guys. I actually did go through the endless debate section for a while but everything there was discussing brands, techniques, and occasionally what was primed. The what and how was easy to find but not so much on the why. Maybe I missed it but I didn't see the reasoning behind why some people primed their mating surfaces. I was looking for something like the easy to trap moisture and extra scratching on assembly. Thanks for that.

I have an additional question on priming.

IF you prime all the parts before you work them, then when you match drill them there is bare aluminum in all the rivet holes. Even if you wait till you have gotten through the dimpling phases, frequently more is done later in the assembly.

So after you have attached the stiffeners or ribs for example, are you then priming again over the whole area? My ability to become OCD here knows no bounds...

Cheers
Andrew

Some people ignore priming freshly drilled holes; bolts and rivets are generally coated or plated (that's why you don't spin the bolt to tighten it if possible, so you don't wear off the coating). However, others set their rivets and bolts 'wet'. That means to freshly coat the fastener with primer and set it while the primer is still wet. Others spot-prime where holes have been drilled; a small brush or Q-tip works for this. You can spray rattle-can primer into a small cup or dish for this purpose. Just like everything else in the primer wars, you'll get lots of advocates and anecdotal evidence for one method or another. In the end, I think they all work fairly well. Being aware of, and regularly inspecting for, corrosion seems to be the most effective technique of all.

Maybe I'm paranoid, but I live in Ireland where is rains most days in the winter..... and the Spring, and the Autumn and.....

Anyhow, right at the beginning I had to learn about corrosion protection too. I ended up going belt and braces. I etch and a two pack epoxy primed everything. For good measure I brushed on JC5 on all surfaces before mating them and put some in all holes before inserting rivets or permanent bolts.

In the winter, the air is so damp, you can watch uncovered tools rust practically in real time. Any time the temperature increases by a degree or two in a relatively short period of time, condensation covers anything and everything inside and out and because its cold, it can take days to dry. If lived where is was dry, I don't think I would have gone to so much trouble, and this would have shortened my build time considerably - not to mention cost. This would apply also if you were confident that the aircraft would spend its idle time (for the duration of its existence) in a temperature controlled environment

Andrew Butler,
RV7 EI-EEO - flying since Feb 2011.
EITT Ireland.