Priming

A place to discuss your primer & paint choices including using paint shops or doing it yourself.
Post Reply
lowaltituderecord
Class G
Posts: 6
Joined: Sat Sep 13, 2008 6:29 pm

Priming

Post by lowaltituderecord »

Ok, I know there is a lot of debate about this but I haven't seen this point brought up about to prime or not to prime. I'm a chemist, actually one that works on polymers, so I got to thinking. If you have 2 coats of paint between mating parts that are riveted together, (one coat on each part) then you will eventually create a loose bond. The paint is not designed to have mechanical strength, and will eventually deteriorate from friction as the parts move over time from both mechanical stress and expansion and contraction from heating and cooling cycles. What's a coat of paint, at least .001", this means that your bonded parts could wind up having at least a .002" gap between them, maybe even more. Then you'll have to fix rivets, that's if you're lucky enough for it to happen in a place where you can repair it. I don't know, do they prime parts on production aircraft? Is corrosion on the interior of the airframe really that much of an issue? Aluminum forms an oxide on the surface that provides a barrier, unlike that of iron oxide.

I'm not going to prime my plane, just give it a good coat of paint on the outside and make sure it's all sealed well, possible repaint the outside on a regular basis.

weezbad
Class C
Posts: 528
Joined: Sun Oct 21, 2007 9:00 pm
Location: darlington s.c.

Post by weezbad »

i really doubt it matters as long as the film thickness is held to a minimum.

i can say hundreds of planes have been built this way as well as cars and although there is not any structural value to the paint i think it is unlikely to work and cause smoking rivets.. the trick is to only use neough to do the job. i used a touch up gun with gravity cup and a .8 tip so there isnt the typical film build but it is very atomized and covered well. i used epoxy also.

i would think that you would have to first exceed the strength of the bond. to cause relative motion. i have disassembled parts that have been on for years and the paint is better here than anywhere else..i suppose hiding from uv helps.

as much as water can shoot between the surfaces i feel the rivet shaft is completely in shear any way and i doubt the rivet driving left much in the way of the shaft to inhibit its job.
william....don't let it beat you down, you are stronger than you think.

lowaltituderecord
Class G
Posts: 6
Joined: Sat Sep 13, 2008 6:29 pm

primer followup

Post by lowaltituderecord »

Good points about priming in automotive construction, but the type of fastener isn't the same. Very different from the low strength, multiple point fasteners, that rivets comprise in the semi-monocoque construction technique. The rivet is very strong in shear applications but the mating materials can move, especially rotationally around the axis of the rivet. How many are careful enough to apply thin coats of epoxy? which actually does have good mechanical strength as opposed to a lot of primer paints. I couldn't find reference to priming interior airframe parts in production aircraft prior to assembly.

weezbad
Class C
Posts: 528
Joined: Sun Oct 21, 2007 9:00 pm
Location: darlington s.c.

Post by weezbad »

well you have to find what sastisfies your needs and allows you to sleep at night. its your arse in then end. all the advise of the folks out here wont mean squat when your headed to earth in a spiraling corkscrew with the wings missing, :o so choose wisely.

as far as rotating around a shaft of the rivet, that isnt the design load. and the very reason so many are used. i would not think there any torsional loads on the adjoined sheets.
like i know what the design load is :lol:

on stuff you feel is of longterm inportantce,,call vans. and look at aircraft that have been in service for years.

of all the structural panels i removed from helicopters they were all primed and none showed evidence of fretting. the fastener does the intended job. YMMV
william....don't let it beat you down, you are stronger than you think.

weezbad
Class C
Posts: 528
Joined: Sun Oct 21, 2007 9:00 pm
Location: darlington s.c.

Post by weezbad »

oh BTW if your not priming with epoxy you are wasting your time.....
run, duck, cover.
william....don't let it beat you down, you are stronger than you think.

lowaltituderecord
Class G
Posts: 6
Joined: Sat Sep 13, 2008 6:29 pm

more priming

Post by lowaltituderecord »

Good info, you say that you saw priming on production aviation airframes. I bet they weren't using "Sherman Williams".

Care to elaborate on the choice of epoxy? Any prior knowledge that was the type of material (epoxy) used in production applications?

One would think there would be rotational forces about the axis of rivets, albeit very minor, but over time this would add up. Do you remember if the parts you saw painted in production applications were also painted on mating surfaces, or painted after assembly?

As you said, as thin as possible is important, there is quite a bit of info out there with regards to maintaining coatings thickness for production applications.

Still, I think I'll be dead by the time my plane rusts out, even without priming, it is aluminum after all. For a 18K (not including my time) airframe I don't know if its worth it.

weezbad
Class C
Posts: 528
Joined: Sun Oct 21, 2007 9:00 pm
Location: darlington s.c.

Post by weezbad »

every single panel on the helis i worked on.. i was in a intermediate maintenance facility so we worked on bell 206's, cobras, hueys, blackhawks, apache's, chinooks. they all were primed with the bright yellow and/or olive zinc chromate. we also were the place depot would ship several depot level repairs to, then they would send out a team to repair them...everything i recall was primed with zinc chromate. i used akzo from aircraft spruce. and nason ful poxy. the axzo is much more durable and extremely resistant to chemicals.
Last edited by weezbad on Mon Sep 22, 2008 8:28 am, edited 1 time in total.
william....don't let it beat you down, you are stronger than you think.

User avatar
TomNativeNewYorker
Class D
Posts: 439
Joined: Tue Mar 21, 2006 9:19 pm
Location: KSAV

Re: Priming

Post by TomNativeNewYorker »

lowaltituderecord wrote:If you have 2 coats of paint between mating parts that are riveted together, (one coat on each part) then you will eventually create a loose bond. The paint is not designed to have mechanical strength, and will eventually deteriorate from friction as the parts move over time from both mechanical stress and expansion and contraction from heating and cooling cycles. What's a coat of paint, at least .001", this means that your bonded parts could wind up having at least a .002" gap between them, maybe even more. Then you'll have to fix rivets, that's if you're lucky enough for it to happen in a place where you can repair it. I don't know, do they prime parts on production aircraft? Is corrosion on the interior of the airframe really that much of an issue? Aluminum forms an oxide on the surface that provides a barrier, unlike that of iron oxide.
I disagree with your statement.

First a little background on me. Since 1985, I have been a structural mechanic for US Naval aircraft, including 14 years working depot level maintenance performing repair, modification, overhaul, and corrosion control and remediation. First off, I have worked on interior areas of numerous components and sub-assemblies to state that upon disassembly of various different parts that over time there hasnt been any noticeable degradation of primer between mating surfaces as you say can happen. On example was a flap assembly for an A-6 Intruder that had major battle damage in Desert Storm in 1991. I had to perform major replacement of the skins and repair/replacement of interior ribs and other interior structural members. First thing I noted upon removal of the skin was it had a manufacture date of January 1961 stamped on the inner surface of the skin. The component was over thirty years old at the time of disassembly, and all interior members were primed. Further disassembly of other interior members revealed that the was no degradation of primer between mating surfaces, nor loose rivets either.

I have seen other components such as engine doors, where interior primers/paints on inner surfaces were missing for whatever reasons. Trust me, unpainted/primed areas are more than likely to corroded in some sort or another. Pure aluminum is highly resistant to corrosion, but your aircraft isnt made from pure aluminum; but alloyed aluminum. Corrosion on aluminum alloys not caught early and allowed to develop can become some nasty looking looking stuff. Another thing is sheet alclad aluminum. It has a very thin layer of pure aluminum for added corrosion resistance on one or (usually)both surfaces depending on the particular specifics of the sheet. People have a tendency to sand these surfaces for numerous reasons such as removing tooling marks made on the surfaces. Remember that sanding or grinding on this pure clad surface removes that added corrosion protection it gives you.

As far as the thickness of paint being .001" thick, I dont know because paint isnt really my trade, but in training we were told that primer should be thin enough that you should be able to read newsprint beneath it.

User avatar
BSwayze
Class C
Posts: 581
Joined: Wed Jun 27, 2007 5:15 am
Location: Portland, Oregon
Contact:

Post by BSwayze »

In case there's still any debate about whether to prime or not to prime, I should tell you that I intially decided not to prime the interior surfaces of the skins on my wings. My thinking was, it's on the inside. And I bought into the reasoning that the alclad would be sufficient after doing a lot of reading and research on the subject. But my technical counselor, Randy Lervold, straightened me out.

Look at the corrosion in these pictures of Jeff Jasinsky's RV-8, especially the closeups, and then make up your own mind. Obviously, the aircraft industry knows what they've been doing after decades of experience, when they prime everything the way they do.

http://www.northshoremktg.com/Jasinsky/ ... index.html

This plane had been stored consistently in a hanger. Yes, the damage is on the outside of the plane, but if it's moisture and/or condensation that causes the problem, the insides of your skins aren't safe either. I'm now priming EVERYTHING.
Last edited by BSwayze on Wed Sep 24, 2008 7:11 pm, edited 1 time in total.

User avatar
TomNativeNewYorker
Class D
Posts: 439
Joined: Tue Mar 21, 2006 9:19 pm
Location: KSAV

Post by TomNativeNewYorker »

On this interior picture

Image

It looks as if underneath the primer beneath the steel nut and cad plated washer stackup there may be dissimilar metal corrosion as well. Hard to tell for sure, but it would be something I would look hard at

Spike
Chief Rivet Banger
Posts: 4013
Joined: Thu Oct 14, 2004 8:40 pm
Location: Baltimore, MD
Contact:

Post by Spike »

Fixed yer image. Remember, linked images need to end in an image format extension unless coming from the rivetbangers gallery.

Spike
http://www.rivetbangers.com - Now integrating web and mail!
Current Build: 2 years into a beautiful little girl

lowaltituderecord
Class G
Posts: 6
Joined: Sat Sep 13, 2008 6:29 pm

priming from experts

Post by lowaltituderecord »

Well, we've managed to extract some information from people with REAL WORLD knowledge of commercial airframe construction. Which is what I was really after. It seems the consensus is that priming is a part of construction and indeed necessary to achieve longevity, with one caveat. Industry recognizes that the choice of materials and the mode of application is crucial.

Once again, doucing your panels with "Sherman Williams" ain't the way to go

Ok I'm convinced, a thin coat of epoxy will be worth the brain damage I will suffer from it's application

Thanks for the input guys.

User avatar
TomNativeNewYorker
Class D
Posts: 439
Joined: Tue Mar 21, 2006 9:19 pm
Location: KSAV

Post by TomNativeNewYorker »

I guess I would throw a wrench into you works if I told you that any parts we installed/replaced, or any type of repair doublers we used; we put a thin layer of sealant on all mating surfaces. Any external doublers also get a bead of sealant on all edges to prevent water intrusion underneath the repair.

Remember, this is on Navy aircraft where corrosion is a serious concern because the likelihood of operations in a high salt atmosphere.

Wanna know about corrosion, ask; I have seen tons of it over the years.

Post Reply