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Another priming thread - Advice please

amerkarim

Active Member
Hi All,

I am now at the planning stages of my RV 10 build and because my airplane is going to spend its life near the ocean, I would like to prime it and protect it as much as possible. I am going to build a large dedicated spay booth for the purpose and have got some large tanks to allow dipping of things in Alodine etc.

My plan was to lightly scuff with a Scotch bright pad, wash with soap and water, then wipe down, dip in Alodine, then once dry, apply a coat of epoxy primer to all the surfaces and parts.

Also to do any dimpling after the primer has full dried.

Regarding the skins, is there any benefit in priming both sides of the skins at one go. I intend to paint the plane once its finished anyway. But it just seems like common sense to me that priming both sides of the skin will be easy in a spray booth and you wont miss any spots, and also protect the outside against corrosion during the build process and build delay. The outside and inside can then receive the final coat of paint once assembled.

Any suggestions, hints, comments or experience would be welcome

Thanks in advance

Amer
 
dimple then paint

Sounds like a good plan. I'd dimple then paint. I'd also let the painter do the outside. You'll get 999 other opinions on both in the next hour or two. :D
 
A couple things..before the war starts..

First off, The skins are "Alclad", a thin layer of pure aluminum which helps protect from the elements, but the edges are most vulnerable. Think of your skin as a sheet of plywood, and what plywood does when wet. I built my -4 to survive conditions of a seaplane, and fully primed inside skins,and all structure. The outside of the skins where primed at the time I applied paint. I took a slightly different approach to application that works very well and creates no mess on the inside of the skins, using 2 part urethane primer overthinned and applied with a smooth foam roller, after all countersinks and dimples are completed. Any bolt hole or rivet hole in the entire plane has been "swabbed" with a Q-tip dipped in primer to coat the edges. The rolling technique will pretty well cover the dimpled backside holes, and swabbing isn't needed. My internal primer coat is very thin, and added very little weight.It may sound a bit overboard, but 38 years of being an aircraft structural specialist for my day job has shown me the reasons why I did it. Be prepared for the war you may have started!!
 
I will second that...

Sounds like a good plan. I'd dimple then paint. I'd also let the painter do the outside. You'll get 999 other opinions on both in the next hour or two. :D

What he said!

I do deburr, dimple, scuff, clean then prime within a 2 hour window... 2K Epoxy pimer. Be ready for this to double your build time!
 
Priming fuse

I primed everything so far. The real hard part is priming the fuselage. There are so many brackets and stuff on the cabin bulkheads that need to be primed before assembly. And if you wish to paint before assembly, this adds another complexity. I?ll get thru it, but it isnt like the wings or tail where you can rough assemble, prime, final assemble and go. The fuse is where a semi permanent paint booth would be worthwhile.
 
My 2 cents

Do you really want primer on the outside of the skin? That will need to be scuffed and re-primed again before painting? Finish paint needs to be applied over an epoxy primer within a short time frame otherwise it needs to be scuffed well before paint is applied.

Also, I’m in favor of priming after dimpling. It’s the edges of the debured, dimpled hole that no longer have the allcad coating. I have done this with SEM primer which is very thin. The one concern is that epoxy primer is thicker and may build up or settle in a dimpled hole. I think if you apply a thin coat of epoxy by tuning down your spray nozzle, it should not be an issue.
 
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If you use a good two part epoxy primer, skip the alodine all together.

I do slow builds. After competing all part fabrication, fit up and dimpling, then it is time to prime. For prep I use Alumiprep and maroon Scotchbrite pads, then rinse with a lot of water. Prime the same day as prep. I alway prime outside with no paint booth so managing priming days around the weather is a factor.

The hardest part in all this is working sections of the build up to the part where you have to rivet something together. Here you have to stop, put the parts in the ?prime bin? and then move onto the next section. At some point you get where you have to prime before moving on. The wings may need two priming sessions. The fusleage (current RV-8 project) needed four. I?m careful to not prime the outside skins or cabin skins that get interior paint other than the overlap areas. They will get done on final paint.

Carl
 
Alodine

I also prime after dimpling. Specifically debur, scuff, dimple, wash with Bon Ani and gray Scotch brute, rinse, dry, prime.
I Alodined the exterior. If I build another, it will be Alodined inside and out before assembly.
I found Alumiprep easier to use as a pretreatment for Alodine. Both Alumiprep and Alodine should be rinsed. Capturing the rinse is a challenge. I made a slice from a shower curtain. It drained into a huge storage tub. The tub was left to evaporate in my storage building till the liquid was almost gone. Transferred it to a stable plastic container and disposed at a Haz Mat facility. There's a Kitplanes Tip section with link on my blog describing it.
 
Regarding the skins, is there any benefit in priming both sides of the skins at one go.

No

The catalyzed primers used with with contemporary paint systems require them to be top coated within a very short time (sometimes only a couple days) or they must be sanded and then recoated with primer again just before spraying topcoat.

More work, more cost, more weight, no benefit.

Etching and alodyn in advance on the outer surfaces is fine.

Considering the health dangers and environmental impacts, and the personal experience of how protected alclad aluminum with even just a very basic sprayed on protection (washer primer) holds up in harsh environments, I personally would skip the etch and alodyn.
 
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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Confirmation

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.
 
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.
 
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