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  #21  
Old 04-28-2017, 09:38 AM
flybill7 flybill7 is offline
 
Join Date: Jan 2005
Location: Severna Park, Maryland
Posts: 446
Default Priming

On my slow-build RV-7 project, finished in 2009, I used self-etching rattle can primer on the non-alcad parts. I scuffed up the parts with scotch-brite, then cleaned them well before spraying. It's hard to believe, but I've been happily flying my 7 for eight years now, and have over 1,000 hours on it. It's always been hangared; the airport is just off the Chesapeake Bay. I have yet to see evidence of any corrosion on the airplane. I can tell you the airplane is going to last a lot longer than I am!
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  #22  
Old 04-28-2017, 10:38 AM
rv7charlie rv7charlie is offline
 
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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I own a -4 that was built in '91. Spent its 1st 20 years in Arizona; rest of the time in Mississippi. Only light discoloration on the inside of the wing bottom skins. (And some light corrosion around chipped paint on the outside...)

This thread is the 1st time I've heard of alclad being called a 'conversion coating'. Supporting docs? I've always been told that it's just a super thin layer of pure aluminum applied to the 2024 alloy.

Charlie
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  #23  
Old 04-28-2017, 10:50 AM
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majuro15 majuro15 is offline
 
Join Date: Feb 2013
Location: Charlotte, NC
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I had my tail cone in a basement for just under a year. Despite having dehumidifiers down there, moisture found a way around the bottom skin and battery mount ribs and cause a lot of corrosion. I had hastily sprayed the Duplicolor self etching primer (not applied well, my fault) and it corroded underneath the primer.

I regret not priming!!

I started using SEM out of a spray can for ease of use on the wings and fuselage. Great product, tough as nails, easily applied.
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  #24  
Old 04-28-2017, 12:39 PM
TFeeney TFeeney is offline
 
Join Date: Mar 2014
Location: Floyds Knobs, IN
Posts: 174
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Tim - I've seen your blog pictures and it was heart wrenching to me.

Can you elaborate on what you think went wrong? Why that one area?
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  #25  
Old 04-28-2017, 03:51 PM
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majuro15 majuro15 is offline
 
Join Date: Feb 2013
Location: Charlotte, NC
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First it was storing it in the basement. However, I will say that the basement was still a much more controlled envrionment than what an airplane will see in it's normal life (outside, rain, hot humid hangar, etc). I had it sitting on the ground with just a scrap of carpet underneath. I believe the metal acted as an attractive surface for moisture to collect and that happened to be the lowest spot.

Second, it was the condition of that piece. The tail kit is old (originally purchased in 2004) and there were more than a few scuffs and scrapes to that area. Any damage to the pristine surface of the Al from Van's can invite corrosion if not properly treated.

Third, was the crappy application of primer. The original builder did not prime for several reasons of his choosing. I went back and forth and decided against it initially. However, after seeing more and more light filiform on pieces, I changed my mind and primed. Initially I used the Duplicolor self etching primer and it's not a quality product in my opinion. I also did NO prepping to the tail cone before I shot it. It was a last minute decision before storing it at the time that "hey, some primer is better than nothing at all" thought. I simply spray painted over a completed assembly, so no real protection from it.

My advice is find a method and product that works for you and do it. It can be done simply, quickly, easily, and cheaply. No point in worrying any more than you need to about the condition of areas invisible once it's built.

Primer isn't a guarantee of no corrosion, it's just a form of insurance that I will not build without!
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  #26  
Old 04-28-2017, 05:36 PM
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JonJay JonJay is offline
 
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Location: Battleground
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Quote:
Originally Posted by majuro15 View Post
I had it sitting on the ground with just a scrap of carpet underneath.
My guess is the carpet did you in, wicking up the moisture in vapor from the concrete.
It is just a guess and not meaning to hinder your other points.

Do not store parts in contact with anything that can absorb or retain moisture.
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  #27  
Old 04-28-2017, 06:06 PM
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Plummit Plummit is offline
 
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Location: Covid Country-SoCal
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Quote:
Originally Posted by JonJay View Post
My guess is the carpet did you in, wicking up the moisture in vapor from the concrete.
It is just a guess and not meaning to hinder your other points.

Do not store parts in contact with anything that can absorb or retain moisture.
To add to that, Concrete has lime in it. It's very possible that the situation was compounded by the carpet and concrete.

~Marc
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  #28  
Old 05-17-2017, 02:13 AM
RV7ForMe RV7ForMe is offline
 
Join Date: Jul 2015
Location: Europe
Posts: 601
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I use 2k chromate epoxy primer that airbus uses on the whole inside.

I have been working on the empenage for a few months. I already have filiform corrosion around rivets on my finished parts on the outside! So I regret not priming the outside as well. I have no way of telling if corrosion is right under the rivets and doing its thing there.
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  #29  
Old 06-21-2017, 12:04 AM
Polar Polar is offline
 
Join Date: Oct 2013
Location: Central Alberta
Posts: 63
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I've also been actively searching for someone who really wishes they had taken the time to prime the inside surfaces. Haven't found a single such story though.

I'm in the 'priming everything, but not sure why' group. It sure adds a pile of work, so far that's all I can see. Hopefully one day I'll find myself incredibly glad I did, although that might be as unlikely as finding someone that is terribly regretful for not priming!
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