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Prime firewall Aluminum parts?

J

jwilbur

Well Known Member
In general I'm not priming my fuse except where skins attach and non-alclad parts. Is it a good idea to prime the aluminum parts that contact the stainless firewall? In general I would think yes since they are dissimilar metals. But I'm wondering about heat (especially in the event of an engine fire) doing bad things to the Zinc-Chromate primer I use: like becoming noxious fumes or the like.

Any thoughts?
 
Yes, prime. Don't worry, if the heat causes problems with the primer, you have other worries.
 
I have been priming with a zinc chromate primer, but mixing small batches was getting wasteful and the fumes caused me to go to rattle can for numerous small parts. Even that was a pain with the fumes inside, so I have tried using alodine for many small parts. I built a work station with 4 vertical PVC pipes 3" ID. One for acid dip, rinse, alodine, and 2nd rinse. 2-2.5 min dip in each step. The tops of the pipes are sealed with expanding plugs for evaporation and contamination. I am a lot happier with this method for the small stuff.

Each his own, but this is working for me. I kinda like the color on FWF.
 
sahrens said:
Here is a lengthy thread about firewall insulation. Although not directly related to your question, buried in the thread are some comments about paint.

You may find it worth your time to read through it and then deciding about the primer.

http://www.vansairforce.com/community/showthread.php?t=47587
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Thanks Scott. I read this referenced thread and based on it, and some other comments, I'm leaning toward skipping the primer. I'm still concerned about the dissimilar metals and corrosion, but I suppose this should be a fairly dry area. Am I missing anything else to either ease or complicate my concerns?
 
Joe

I did prime the aluminum angle on the inside of my firewall and now wish I had not. I had not read the thread before hand. If I had I would not have primed those parts.

I am getting dangerously close to re-starting the primer wars, but here we go. I just sold my 1965 Cherokee 180. It did not have primer on the inside, at least not that I could see. I could see a lot of aluminum that was not primed. From my perspective your location, its weather, is the aircraft going to be inside a hangar or tied down on the ramp; all have an impact on an aircraft's development of corrosion.

On your aircraft you need to build it to your comfort level. On mine, I would have been fine without it. But folks will disagree with me.

Good luck with your build.
 
Thanks Scott.

Your comments are reasonable. I've been priming areas I felt would be difficult to access or where dissimilar metals attach. In this case, even though there are dissimilar metals I'm going to skip the primer. The airplane will be hangered so I'll trust the alclad to be enough corrosion protection.

Thanks for pointing me to that thread. It was extremely helpful.
 
I must admit I think I skimmed that thread too quickly because I didn't see the explanation of why it's better to leave the aluminum raw where it is riveted to the firewall. Can you remind me what the reason was? :)
 
Master Designs said:
I must admit I think I skimmed that thread too quickly because I didn't see the explanation of why it's better to leave the aluminum raw where it is riveted to the firewall. Can you remind me what the reason was? :)
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Someone asked the question but it wasn't answered by anyone. My reasoning is based on three things:

1... The smoke mentioned earlier in this thread (post #4). In my original question I indicated concern about Zinc Chromate generating noxious fumes, etc. The thread you mentioned drew my attention to heat on the inside of the firewall being something to think about.

2... Upon researching Zinc Chromate I ran across some articles showing that higher temps reduce "chromated Zinc's" corrosive protection effectiveness. I saw a bunch of articles discussing this. Here is one example. By the time it gets to 212 degrees F, it has no effectiveness at all. Now, I am making an assumption here that "chromated zinc" is related in someway to Zinc Chromate (granted, this assumption may be wrong). In any case I don't know how hot the firewall will get under normal operation but these articles gave me another thing to consider. That is, will Zinc Chromate primer have any benefit at all when coating aluminum against the firewall?

3... Given that Van's doesn't say priming is important on the alclad parts and so many production airplanes are out there without primer, my points #1 and #2 provide me enough question and doubt to justify skipping it in this case.

Perhaps my reasoning falls short; I'd prefer to have a clear definitive answer: Is Zinc Chromate good next to the firewall or is Zinc Chromate bad next to the firewall? - but at some point you just have to make a decision and go with it.
 
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jwilbur said:
Thanks Scott.

Your comments are reasonable. I've been priming areas I felt would be difficult to access or where dissimilar metals attach. In this case, even though there are dissimilar metals I'm going to skip the primer. The airplane will be hangered so I'll trust the alclad to be enough corrosion protection.

Thanks for pointing me to that thread. It was extremely helpful.
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Just remember that the firewall angles are not alclad; they're bare 6061 aluminum.

Master Designs said:
I must admit I think I skimmed that thread too quickly because I didn't see the explanation of why it's better to leave the aluminum raw where it is riveted to the firewall. Can you remind me what the reason was? :)
Click to expand...

IIRC, the reason is that if you have a fire, the primer can produce "toxic" smoke. I didn't think of that when I built my firewall and primed them per plans and several others' build logs I've read. If I had it to do over I might alodine instead of prime, but I'm not going to dissasemble my firewall at this point to change it. I'm with others in that if you have a fire severe enough to smoke the primer, you have MUCH bigger problems than a little primer smoke in the cockpit.
 
longranger said:
Just remember that the firewall angles are not alclad; they're bare 6061 aluminum
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Hmm. I'm pretty sure all the parts for attachment to the firewall in my kit were alclad. I can say for sure they were all covered in blue plastic. ... My understanding was anything with the blue plastic was an alclad part ???
 
Yes, even the angles are alclad. On my -6A, the angles were fabricated from non-alclad angle stock. For the -10, Vans has used formed sheet alclad to make their own angles. Still, it's pretty thick material, which means the sheared edges are exposing a lot of surface. Also, the weldments are powder-coated but after drilling them and scratching the coating with mating parts, etc., I felt better cleaning them of cutting oil and touching up the holes.
 
jwilbur said:
Hmm. I'm pretty sure all the parts for attachment to the firewall in my kit were alclad. I can say for sure they were all covered in blue plastic. ... My understanding was anything with the blue plastic was an alclad part ???
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Thanks for the correction. I wil now attempt to remove my foot from my mouth:eek:. I had no idea the firewall angles in a -10 were alclad. On a -7 the gussets are alclad, but the angles are not. I primed them all.
 
It certainly is a trade off. The potential of toxic fumes (slight) versus corrision (also slight over the short run.) As I said earlier, my angle aluminum is primed, but the firewall is not. I do not believe I would prime those angles knowing what I know now.

But that is the advantage of building our own aircraft. Choosing which way to go.
 
I primed my angles on the firewall. My reasoning was, over time, there will be dissimilar metal corrosion between the aluminum and S.S., but I may never have a fire in the engine. I should note that I plan on installing a fire/heat barrier on the forward side of the firewall (fiberfrax and S.S. foil) to help in the case of a fire.
 
J Twilbeck said:
... My reasoning was, over time, there will be dissimilar metal corrosion between the aluminum and S.S., ... .
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Just to clarify, there will be corrosion if an electrolyte is present. Contact alone between dissimilar metals won't result in corrosion.
 
jwilbur said:
Just to clarify, there will be corrosion if an electrolyte is present. Contact alone between dissimilar metals won't result in corrosion.
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This is true. Humidity condensing on a cool skin, rain, damp hangars, salt water environment, dirt and grime absorbing moisture, etc.
 
For the record, I decided to put a coat of primer only on the AL surfaces that contact the firewall (not the whole aluminum part). As I prepared to rivet, I realized a lot of alclad coating was removed with the countersinking of so many holes. My oscillating opinion swung the other way.

Thanks everyone for your helpful comments.
 
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