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  #11  
Old 05-31-2018, 02:11 PM
StressedOut StressedOut is online now
 
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Location: Fullerton, CA
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I'm the type of person who isn't fully satisfied with only knowing the "how" of a procedure. I need to know the "why" as well. I'm no expert in the field of metal prep and prime, but I do have access to the experts at work.

I asked our materials & process engineers to explain the why of metal prep. It boils down to one thing. A clean surface (no oils, evaporated solvents, dirt, etc.) completely free of oxidization. It's the oxides on the surface that make it difficult for primers to adhere. That's pretty much the goal.

There are several ways to achieve this. Pre-wash the part with plain old soap and water. Dawn dishwashing detergent works well for the dirt and oils. Oxides are removed by manually abrading the surface with Scotchbrite and Prekote to scrape it off, or you can bath the part in an acid etch (Alumiprep 33, now called Bonderite C-IC 33 AERO which is nothing more than phosphoric acid). I suppose you could also sandblast the part, but I've never heard of anybody doing that.

The test to see if you've succeeded in removal of the oxides is simple. You must get a water break free surface. What does that mean? Aluminum oxides are hydrophobic. That is, water will bead up on the surface. You want to see the opposite of that. Water that spreads evenly over the surface is the sign that all of the oxides are removed. The surface has now become hydrophilic. Primer sticks well now.

Once you achieve a water-break free surface, you can dip the part in an Alodine (Bonderite) bath for a few minutes. This is called a chemical conversion coating. It applies a protective layer on surface. This step is optional if you're going to prime, and as Scott mentioned the Alodine is environmentally nasty stuff. You need to properly dispose of the rinse water and used solution. Don't dump this in the ground or down the drain. It's full of chromates. Did you ever see the movie Erin Brockovich? Same stuff.

Whatever method you choose for prep, you must apply primer within a short window of time. Oxides begin to reform on the surface almost immediately. The Prekote instructions say to shoot primer within 24 hours. I heard others say less than that, but I've not seen a source for that information. If you don't apply primer before the oxides reform you must repeat the process again.

Personally, I've use the Prekote method. I'm going to try Alumaprep and Alodine next time because manually abrading, for me at least, takes a long time, is physically exhausting and very boring. My shoulders and back are screaming at me the next day.
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Art Jackson
RV-14A Kit#140433
Completed: Vertical Stab/Horizontal Stab
Scrapped: Rudder
Working on: Empennage (Elevator)
Construction log - mykitlog.com/ajackson
Dues paid on 2 November 2017
Member of EAA Chapter 92 (KCNO)

Last edited by StressedOut : 05-31-2018 at 03:15 PM.
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  #12  
Old 05-31-2018, 08:57 PM
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wirejock wirejock is offline
 
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Quote:
Originally Posted by StressedOut View Post
I'm the type of person who isn't fully satisfied with only knowing the "how" of a procedure. I need to know the "why" as well. I'm no expert in the field of metal prep and prime, but I do have access to the experts at work.

I asked our materials & process engineers to explain the why of metal prep. It boils down to one thing. A clean surface (no oils, evaporated solvents, dirt, etc.) completely free of oxidization. It's the oxides on the surface that make it difficult for primers to adhere. That's pretty much the goal.

There are several ways to achieve this. Pre-wash the part with plain old soap and water. Dawn dishwashing detergent works well for the dirt and oils. Oxides are removed by manually abrading the surface with Scotchbrite and Prekote to scrape it off, or you can bath the part in an acid etch (Alumiprep 33, now called Bonderite C-IC 33 AERO which is nothing more than phosphoric acid). I suppose you could also sandblast the part, but I've never heard of anybody doing that.

The test to see if you've succeeded in removal of the oxides is simple. You must get a water break free surface. What does that mean? Aluminum oxides are hydrophobic. That is, water will bead up on the surface. You want to see the opposite of that. Water that spreads evenly over the surface is the sign that all of the oxides are removed. The surface has now become hydrophilic. Primer sticks well now.

Once you achieve a water-break free surface, you can dip the part in an Alodine (Bonderite) bath for a few minutes. This is called a chemical conversion coating. It applies a protective layer on surface. This step is optional if you're going to prime, and as Scott mentioned the Alodine is environmentally nasty stuff. You need to properly dispose of the rinse water and used solution. Don't dump this in the ground or down the drain. It's full of chromates. Did you ever see the movie Erin Brockovich? Same stuff.

Whatever method you choose for prep, you must apply primer within a short window of time. Oxides begin to reform on the surface almost immediately. The Prekote instructions say to shoot primer within 24 hours. I heard others say less than that, but I've not seen a source for that information. If you don't apply primer before the oxides reform you must repeat the process again.

Personally, I've use the Prekote method. I'm going to try Alumaprep and Alodine next time because manually abrading, for me at least, takes a long time, is physically exhausting and very boring. My shoulders and back are screaming at me the next day.
Nice to know at least two of us have the same opinion. Notice the second post on my blog.
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http://wirejockrv7a.blogspot.com
wirejock at yahoo dot com
Donated 12/01/2017. Plus a little extra.
RV-7A #73391, N511RV reserved (1,800+ hours)
HS SB, empennage, tanks, wings, fuse, working finishing kit
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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  #13  
Old 05-31-2018, 11:17 PM
StressedOut StressedOut is online now
 
Join Date: Jun 2017
Location: Fullerton, CA
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Quote:
Originally Posted by wirejock View Post
Nice to know at least two of us have the same opinion. Notice the second post on my blog.
Very nice blog Larry! I can see I'm going to spend some serious time going through it.


I'm very fortunate to have experts in every aspect of aircraft building within a dead cat swing of my desk at work. What I don't know I can find out quickly from someone I trust.
__________________
Art Jackson
RV-14A Kit#140433
Completed: Vertical Stab/Horizontal Stab
Scrapped: Rudder
Working on: Empennage (Elevator)
Construction log - mykitlog.com/ajackson
Dues paid on 2 November 2017
Member of EAA Chapter 92 (KCNO)
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