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Did you prime your wing spar?

Danny Quist

I'm New Here
I'm thinking that I should prime it but since it is alodine coated it seems like this might not be the best idea. Is this just another way to add weight or is it a good idea? Currently I've spot primed all the parts where I've countersunk or exposed raw aluminum. Is that sufficient?

Thanks in advance!

Danny
 
Your wing spar should be (is) anodized - a nice gold color. No need to prime that. I'd say, definitely prime the countersink areas or exposed aluminum, though.
 
Mr. Boeing says

Alodine and anodized are good but considered conversion coatings. In the Boeing world all parts also need a primer. The anodized finish alone is not considered the end finish for primary structure. With that said, what weight would primer add? How hard is it to inspect the spar for corrosion or repair corrosion? What environment are you going to fly in? How long are you going to live? The last question means, every airframe has a limited life; how long do you need it to last? Priming can't hurt, but it may not add significantly to the service life of the airframe? I have not decided yet my self. Aerospace standards do not allow the anodized finish as the prime defense against corrosion according to Boeing experts. G
 
Ask Van's

Ask Van's about this...see what they say. My money says they'll tell you to leave it alone, and I'll bet they'll cite two points:

First and foremost, clearances. Is the spar and center section designed to accommodate TENS of thousandths additional crap in between? I don't believe so. Maybe on earlier models, but not the CNC-fab'd stuff. Even a thin layer of primer is really 4x as thick when you get done with it -- one layer on each side of the spar, and of course one layer on each side of the center section (prime the anodized spar, you'd obviously carry the philosophy over and prime the center section as well, right?). Four .004" layers = .016". See if you can jam a slice of .016" aluminum in there when the wings are mated...na-ganna-da! Prime those spars, and when you go to fit your wings, boy oh boy those suckers are gonna be tight. I'll bet Van's will agree with this...leave those anodized spars alone! At a minimum, I would not mess around with the root stubs that mate with the center section. And at that point, why bother at all?

My hangarmate primed his -7A wing spars, and I'm dreading the day when I have to help him mate his wings...I predict we'll be solvent'ing the primer away. And at that point, the anodized surface has theoretically been compromised due to the self-etching primer!

The second point is -- look at quickbuild wings. Primed spars? No-sir. Anodized and left alone.

Sure, prime the exposed aluminum after countersinking, and I'm not saying you shouldn't prime adjacent mating surfaces, but the spars & center section are a different story IMHO. Just my 2 cents.

Give Van's a call if you think I'm off base here, but I recommend NOT priming your anodized spar.

)_( Dan
RV-7 N714D
http://www.rvproject.com
 
Info

I am building my wings now and I believe that the instructions said that there is no need to prime any part of the spar unless it is the areas that you have removed the anodizing. N-7BA
 
Build it per plans

I agree with Dan. However you could paint many parts of the spar, spar caps with out danger of tolerance build up and fit problems. Also one light coat would do, and not prime in the interface between spar and bulkhead. G
 
Last edited:
If your primer coats are 0.004 inches thick, then they are WAY too heavy.
The Mil-Spec for epoxy primer is only 0.0005 to 0.001 thickness. It should be somewhat transparent when dry.
Build-up of thickness should not be a part of this descision... apply the appropriate material correctly.

More details are in the DETCO guide for aircraft painting - U-1201 is the primer Mil-P-23377

http://www.detcosterling.com/slacguide.htm

Sterling used to give a healthy discount to EAA members... don't know if that is still the case, but is worth asking for if you order from them...

gil in Tucson
 
Thick primer

Thanks for the spec, Gil. I've seen some pretty thick applications of primer out there. Some people seem to think it goes on like paint. :cool:

)_( Dan
RV-7 N714D
http://www.rvproject.com
 
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