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  #1  
Old 05-24-2021, 10:07 PM
mags591 mags591 is offline
 
Join Date: Jul 2020
Location: Bowie, MD
Posts: 26
Default Priming "mating" surfaces

Hello,
I am curious to why if you chose to prime, many times people only prime the "mating" surfaces and don't prime the entire thing? Is there a benefit to it? I will be starting my build and live in Maryland right on the Chesapeake bay (W29).
I appreciate all the comments.
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  #2  
Old 05-24-2021, 10:30 PM
DrillBit's Avatar
DrillBit DrillBit is offline
 
Join Date: Nov 2007
Location: Pleasanton, CA
Posts: 211
Default

Weight. Time. Money.

I primed 6061 parts and flying surfaces. Saved all three. 😁
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  #3  
Old 05-24-2021, 11:26 PM
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GeoffP GeoffP is offline
 
Join Date: Sep 2018
Location: NSW, Australia
Posts: 206
Default

I've primed mating surfaces on skins etc. The logic I had was once those parts are riveted together, I can't easily access them again. I'm still building so it could be 10-20 years before I know if I was right or wrong.
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  #4  
Old 05-25-2021, 03:30 AM
KayS KayS is offline
 
Join Date: Jul 2009
Location: lake constance
Posts: 387
Default

i primed every surface inside the plane and wouldn't do it again. a lot of weight, cost and effort. when i look at some now 12 year old super shiny alclad surfaces which were not primed, i really start to question my strategy.

if i would do it again i would only prime the non-alclad parts and all the mating surfaces. an aircraft that doesn't live in some extreme environment should be good to go with non-primed alclad surfaces.

just make sure the alclad is clean, with no fingerprints, dirt and stuff before you rivet them together.

Last edited by KayS : 05-25-2021 at 03:52 AM.
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  #5  
Old 05-25-2021, 05:08 AM
fixnflyguy fixnflyguy is offline
 
Join Date: Jan 2006
Location: Winston-Salem, N.C.
Posts: 1,407
Default 100% Primed mine

I have been in the heavy jet overhaul business for 42 years, mostly as a structures specialist, dealing with corrosion control and aging aircraft extensively. I know most GA aircraft came unprimed, and I also as an IA keep up with a bunch of them too. Corrosion is no ones friend..especially in costal regions. When I built my -4 years ago, I didn't even question priming, as its in my blood to do it. I even primed the inside of every hole..yep. I use 2 part fluid resistant primer (Akzo and the likes), overthinned for extremely light weight penalty. Small parts are sprayed, inner skin surfaces rolled with a smooth foam cabinet roller. I can bet I added very little weight, and I know nothing will ever corrode. To me, I spent very little additional time, as I would prime large batches of prepared parts. All bolt holes were swabbed with a Q tip to cover the inside edges which is the root of all corrosion evil. Overkill maybe, but I couldn't sleep with out doing it this way.
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  #6  
Old 05-25-2021, 05:37 AM
KayS KayS is offline
 
Join Date: Jul 2009
Location: lake constance
Posts: 387
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was there ever a ceasefire agreement in primer wars?
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  #7  
Old 05-25-2021, 06:34 AM
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mike newall mike newall is offline
 
Join Date: Jan 2005
Location: Yorkshire, England
Posts: 2,159
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The UN has been consulted.......

My take is this.

All internal surfaces are primed with a decent etch primer.

Anywhere there is a skin overlap, I Scotch the outside mating surface an inch or so from the edge and quickly run a rattle can etch down it and try cleco up asap. That way, the two surfaces will tend to soften together and help the condensation from starting tinworm later on.
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  #8  
Old 05-25-2021, 06:41 AM
lr172 lr172 is online now
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,334
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while I primed all parts, I believe the reason that mating surfaces are more susceptible to corrosion is that the mating areas will tend to hold moisture longer than exposed surfaces, due to reduced air contact. You can experiment. Take a piece of mild steel and place a second piece of steel half its size on top of it. Pour some water on it and come back the next day and observe. I have observed this several times with steel in my projects.
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Last edited by lr172 : 05-25-2021 at 06:44 AM.
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  #9  
Old 05-25-2021, 07:20 AM
Freemasm Freemasm is offline
 
Join Date: Nov 2017
Location: Orlando
Posts: 417
Default For those who wish to do so

Someone mentioned Q-tips. If you are going to use primer inside of fastener holes (some applications call for such like Hi-Loks, depending on material), finger nail polish bottles work great for single part primers. If using a high solids primer, find said bottle that still has the mixing BBs in it. A thought for those who wish to do so. I do between dissimilar metal applications. Here in humid AF Florida, the air is full of catalyst to promote all types of corrosion.

Different subject = fog, fog, fog your airframe.

Last edited by Freemasm : 05-25-2021 at 08:53 AM.
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  #10  
Old 05-25-2021, 07:55 AM
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wirejock wirejock is offline
 
Join Date: Oct 2005
Location: Estes Park, CO
Posts: 4,540
Default Primer method

Quote:
Originally Posted by fixnflyguy View Post
I have been in the heavy jet overhaul business for 42 years, mostly as a structures specialist, dealing with corrosion control and aging aircraft extensively. I know most GA aircraft came unprimed, and I also as an IA keep up with a bunch of them too. Corrosion is no ones friend..especially in costal regions. When I built my -4 years ago, I didn't even question priming, as its in my blood to do it. I even primed the inside of every hole..yep. I use 2 part fluid resistant primer (Akzo and the likes), overthinned for extremely light weight penalty. Small parts are sprayed, inner skin surfaces rolled with a smooth foam cabinet roller. I can bet I added very little weight, and I know nothing will ever corrode. To me, I spent very little additional time, as I would prime large batches of prepared parts. All bolt holes were swabbed with a Q tip to cover the inside edges which is the root of all corrosion evil. Overkill maybe, but I couldn't sleep with out doing it this way.
That's a very interesting process. Thinned Akzo applied with a roller. I bet it's well protected with a very thin layer and minimal labor. You might consider writing that up to the Tips thread.

I sprayed P60G2. Very thin. I calculated the weight but it was so small the number didn't make sense. Less than a pound. All surfaces plus any overlapping exterior fay services.
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Disclaimer
I cannot be, nor will I be, held responsible if you try to do the same things I do and it does not work and/or causes you loss, injury, or even death in the process.
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