This post is a series of random notes on priming the interior of aircraft structures to prevent corrosion. Its purpose is to document my early research findings on the subject. As I move from the planning phase into the actual build, I’ll put together a follow-up entry on my chosen priming method and results.
According to Wanttaja, “Every item made from 2024 or 7075 alloy should be cleaned, etched, alodined, and primed before assembly.”
- Cleaning
- Etching
- Alodining (Chemical Conversion)
- Priming
- Sealing the Faying Surfaces
You could probably condense the first three processes and just call them “Surface Preparation.”
Some Initial Thoughts
There seems to be a sort of continuum that defines the “RV Internal Priming Theory” spectrum, which runs from “No Primer” on the one end, to “Scotchbrite/Alumiprep/Alodine/AKZO” on the other. If it can be reasonably argued that the AKZO, etc. method offers the most thorough protection, and if a case can also be made for doing no priming whatsoever without the airframe turning to dust in your hands, then it’s safe to say that several adequate “middle roads” exist.
I’m finding several builders whose process is similar to this:
- Abrade with maroon Scotchbrite pad, household detergent and water.
- Degrease with MEK, acetone, PPG DX330, Coleman lantern fuel, or something similar.
- Prime using whatever brand, toxicity, and style of application best suits the builder.
I think that’s what I’m going to do. But just for kicks, let’s go over the range of possibilities.
Cleaning
Cleaning means the removal of surface grime and corrosion. Exposed aluminum surfaces quickly form an oxidation layer, which, ironically, helps avoid further oxidation, but it needs to be removed before primer is applied. To break the corrosion and lift surface grime, try a mildly abrasive household cleaner (Dawn?) and Scotch-brite pads. When finished, wipe it down with MEK or acetone to get rid of fingerprints.
Sherwin Williams R1K213, Martin Senour 6387, PPG DX330, DuPont 3939S, BASF 901
First of all let’s clear up a few things; Lacquer thinner, acetone, MEK, and enamel or urethane reducers are NOT surface cleaners.
Then, list specific product options usable for each step, including (especially) enviro-friendly solutions like Citra-Safe/d-Limonene instead of MEK or acetone.
As a straight solvent, d-Limonene can replace a wide variety of products, including mineral spirits, methyl ethyl ketone, acetone, toluene.
Etching
Primer doesn’t like to stick to the smooth, shiny aluminum surface, so we etch the components by hand, with Scotchbrite, or with a weak acid solution called an etchant.
Scotchbrite Pads
- Type I – Aluminum oxide (minimum of 94% ALO3 and a hardness of 9.4 Moh’s) Color: Maroon.
- Type II – Natural silicon dioxide (SiO2) (hardness of 6.8 to 7.0 Moh’s). Color: Tan.
- Type III – Silicon Carbide (minimum of 97% SiC and a hardness of 9.6 Moh’s) Color: Gray.
Aluminum oxide, silicon dioxide, and silicon carbide are classed as ceramics. Although one aerospace manufacturer specifies aluminum oxide pads for cleaning before chemical film treatment, I have it on good authority that all three ceramics are okay for aluminum.
Scotch-Brite™ Hand Pads 7447 or at Cleavland Tool: Scotch-Brite Red Hand Pad
Chemical Conversion Coatings (Alodining)
Alodining chemically changes the outer layer of aluminum to make it more corrosion-resistant. The end result is a golden-brown tint to the aluminum.
Although I believe the best corrosion control is produced by starting with a chemical conversion coating, very good corrosion control can be had by using two coats of corrosion inhibiting primer in place of the chem film. — Lee H. Erb, EAA Chap 1000 Det 5, Arlington TX; EAA Chap 34, June 1997
“The simplest way to prep the aluminum is to wet abrade with Scotchbrite and water, followed by solvent cleaning with acetone, or MEK, or toluene. The alternative is alumiprep 33 acid plus alodine 1201 conversion coating. This is NOT preferred because of the hazardous chromates in alodine.
In either case, before priming, it is essential to remove the thin oxide layer that forms on aluminum. Scotchbrite or acid etch will accomplish this.”
– John Griffin, Sales Manager Akzo Nobel Aerospace Coatings
That’s all I need to hear to skip the alodine process. My new process will be to wipe the parts down with acetone or laquer thinner to remove the oils, then alumiprep, and finally prime. That seems to be the least hazardous method to using AKZO. Source.
PreKote®
This cool product might just take the place of the first three steps altogether, and is non-toxic to boot.
PreKote can be used on a wide range of surfaces including aluminum, steel, magnesium, titanium, CRES, galvanized surfaces, composite and plastic surfaces. Consistent, high performance results are achieved with a variety of paints and primers.
PreKote is non-toxic, non-corrosive, non-flammable, CFC free, ODS free, odor free, and readily biodegradable upon disposal. PreKote eliminates the forced air containment suits required for traditional chromated conversion coatings. Simple rain gear and goggles are all that is needed.
“For airframe construction, I’m going to try AKZO purchased from Aircraft Spruce and a product called PreKote as a pre-prime surface etch prep. For mixing and cleanup considerations, I’m going to try the 3M Paint Preparation System.” — Scott Card
Priming
Generally, it is accepted that two-part epoxy primers provide the best corrosion resistance. However, they are expensive, toxic, heavy and dry slowly all of which makes them problematical for the home builder. If you can tolerate those issues and want your RV to be in good shape when your grandchildren inherit it, they may be “best” for you. — Section 5A, Van’s RV-8 Preview Plans
Self-Etching Aerosol Primers
- Sherwin Williams GBP-988
- NAPA 7220
- MarHyde (light gray)
- Tempo A-880 (gray)
- SEM Self-Etching Primer
SEM Self-Etching Primer (Amazon.com)
Randy Lervold’s notes on interior priming on his RV-3B:
In deciding on the SEM for interior use I did consider the more thorough processes such as acid etch, alodine, and epoxy primer. In the end however, I decided that the materials listed above represent the best balance between the time investment and the protection they afford. I did informal testing with most of the commonly available self-etching rattle cans and found the SEM to be at least as durable as any of them, but left a nicer finish. [emphasis mine] — Randy Lervold
Sealing the Faying Surfaces
Fayed Surfaces of Similar metals – Seams and joints that possess fayed surfaces of similar metals shall be protected, at a minimum, by application of primer coating to each surface.
Just before riveting two pieces together, brush (spray or lightly wipe) the faying surfaces with primer, then install the rivets “wet.” If the rivets are inside and not exposed to weather, the “wet” can be primer. If one end of the rivet will be in the “weather,” then use sealant. Use a “Q” tip to put in a lot of primer just before placing the rivet in the hole. If you are using sealant, experiment a bit so that you don’t get too much squeeze-out. It is a skill.
Why the rivets should be installed wet: After drilling the holes of both sheets, the holes need to be deburred. We always used a large twist drill. The deburring leaves a small chamfer which may or may not be filled with rivet when it is installed. Installing “wet” pretty well ensures that there will be no void for electrolyte (water) entrapment.
Joe
After reading the MSDS on Alumiprep 33, I’m reluctant to use it around the home, yard. I’m trying to determine if Prekote is a substitute just for Alodine 1201 chromate conversion coating or if it substitutes for both the Alumiprep 33 and Alodine applications. Have you been able to determine that? Does Prekote clean and coat?
February 3, 2009 at 3:02 PM
Brian
Joe, thanks for dropping by. I have yet to use Prekote, so I’m certainly no expert, but as I understand it, it acts as a cleaner and etchant, so it would seem to be a substitute for the Alumiprep surface prep process. Although the literature talks about it as an alternative to Alodine, I don’t believe it creates a chemical conversion coating of any kind — I could be wrong.
I’m with you when it comes to minimizing the chemical hazards around the shop, which is why I’m inclined to skip alodining altogether. If Prekote works as a good surface prep for some type of two-part primer, then I’ll probably use it.
Scott Card is using a Prekote prep and AKZO epoxy primer on his RV-8 project, and seems happy with the results. You might want to get his opinion on the process.
February 3, 2009 at 10:57 PM
Brian
Now that I’m getting closer to the actual build, I’m leaning toward using the SEM Self-Etching Primer for the interior surfaces, for reasons outlined above. I like the idea of getting adequate protection while also minimizing the toxicity and hassle of application.
August 20, 2009 at 7:58 PM