Today we began work on the vertical stabilizer, which — being very similar to the horizontal stab — went much more quickly than we’d anticipated.

Dad had already done most of the prep work on the VS parts, including edge deburring the ribs and finishing the rear spar doubler on the Scotch-brite wheels. Dad reported that the 1″ diameter 3M Cut & Polish Wheel from Cleaveland Tool (mounted in the Dremel) made deburring the big lightening holes a breeze.

So, we blasted right into match-drilling the understructure and the skin, including the rudder attach brackets on the back of the spar. The VS skin is much easier to get on and off the ribs for some reason, maybe due to the fact that there’s no center nose rib.

After drilling, we flowed right into deburring and dimpling, and — even though we planned ahead of time not to dimple the fairing attach holes on the lower edge of the stab — we got a little ahead of ourselves and dimpled a few holes that didn’t need it.

We also dimpled and countersunk the holes at the bottom of the rear spar — the ones that butt up against the fuselage attach point and thus must be flush riveted. Once again, the microstop countersink mounted in the drill press made short work of this task.

Next, we moved right into the surface prep and priming process, exactly as we did on the horizontal stab. This time however, we had the benefit of good weather and temps in the upper 60s. We decided to stop with only two coats of the SEM primer this time; it saves weight and primer, and leaves just about the same finish as the three coats we did the first time.

So in a single day’s work (thanks to the great preparation by Dad) we managed to knock out a sizeable chunk of the vertical stab. We should be able to finish up the riveting in just a few more hours.

SEM 39683 Self-Etching Primer (Gray)

Well, today was the big day — priming day. I’ve been thinking about how I’d do this properly for nearly a year, so it was great to finally get started. I had read just about everything I could find on priming and corrosion control, and while I had initially considered going all out with alodine and a two-part epoxy primer, in the end I followed the logic of builders like Randy Lervold, who determined that a good self-etching primer was the best compromise between corrosion protection and ease of use.

So I ordered two rattle cans of gray SEM Self-Etching Primer from Amazon, and decided to see how it worked out. Our prep and painting process is detailed below.

Surface Preparation

(1) Dry abrasion with maroon Scotch-Brite pad to remove the Alclad coating, smooth surface anomalies, and give the part some “tooth” for the primer. The part should have a dull, matte gray finish.

(2) Abrasive wash and rinse using maroon Scotch-Brite pad, hot water, and Dawn dish liquid to remove surface contamination and aluminum particles left from step one. The hot water rinse should “sheet” off the surface, rather than beading up, indicating that it has been properly cleaned.

(3) After drying, a final wipedown with a wet cloth soaked in naphtha, followed immediately by a wipedown with a clean, dry cloth, to remove any residual oils and contaminates left from the previous step. Impermeable gloves must be worn from this point on, to keep skin oils away from the parts.

Primer Application

It’s a good idea to read the MSDS before using any product like SEM self-etching primer. We picked up a pair of 3M 6000 Series respirators with 6001 organic vapor cartridges for lung protection and all-around Darth Vaderness. Eye and skin protection are recommended as well.

Temperature, moisture and dust control are important for any painting process. As you can see, we had none of the above. The SEM has such a strong odor that we couldn’t bear to set up any kind of paint booth indoors, but temperatures outside today were in the single digits. Our solution was to spray each part individually outdoors — a process taking roughly 90 seconds per coat — then rush it back inside for drying before the primer started to freeze. Ideal? Hardly. But the SEM dries so uniformly that it seemed to work okay.

The SEM dries well enough for recoating in five to ten minutes, and we put on three coats per part. We also prepped and primed the rivet lines on the inside of the skins, since we’d scratched up the Alclad so badly during layout and drilling. All in all, we used almost all of two 15.5 oz. rattle cans for the horizontal stab.

I’m very happy with the end result, although we got a run or two and a few spots that could have used a bit more coverage. As you can see, even with the poor conditions, the SEM is very forgiving of amateur painting techniques. Next up, riveting!

3M Paint Preparation System

Thanks in part to this glowing review from Scott Card’s build log, 3M’s Paint Preparation System is high on my “To-Buy” list of priming accessories. This short video gives a quick rundown of the ins, outs and what-have-yous.

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:

1. Abrade with maroon Scotchbrite pad, household detergent and water.
2. Degrease with MEK, acetone, PPG DX330, Coleman lantern fuel, or something similar.
3. 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.

Source

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

Stewart Systems EkoEtch

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)

• 39683 Gray, Aerosol
• 39684 Gray, Quart
• 39681 Gray, Gallon

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.

Source

Cabin Interior

• Sherwin Williams Jet Flex
• DeVilbiss GTi 620G Millennium HVLP spray gun

References

• AvWeb.com Corrosion Control
• Muroc EAA 1000 Corrosion Control Articles
• Aviation Maintenance Technician Handbook, Chapter 6