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Is Self Etching Primer Alone Sufficient for for Internal Aluminum Parts?

apivovarov

apivovarov

I'm New Here
We are currently building an RV-12 and have been applying Rust-Oleum Self Etching Primer to all internal aluminum parts.

However, I’m not sure this is the best approach...

As I understand it, Self Etching Primer is not a protective coating on its own. It chemically etches the aluminum surface to promote adhesion, but by itself, it doesn’t offer long-term corrosion resistance. In fact, it removes the aluminum's natural oxide layer - which normally provides some protection - potentially making the metal more vulnerable to oxygen and moisture if left uncoated.

I’m now considering applying a topcoat over the primer - such as Rust-Oleum Automotive Gloss Acrylic Enamel - for added protection.

Can anyone comment on this? What would be the recommended approach here, without going overboard?
 
I’ve been using NAPA 7220 self etch primer on all internal parts for my 7A build. I’m about half way through the fuselage now and I’m switching to Stewart System Ekoprime for the cockpit primer. I’d have no reservations about using self etch on areas of the plane that won’t see daylight. I’m only switching now for something that will hopefully be a little more resilient to high traffic areas.
 
dwollen89 said:
I’ve been using NAPA 7220 self etch primer on all internal parts for my 7A build. I’m about half way through the fuselage now and I’m switching to Stewart System Ekoprime for the cockpit primer. I’d have no reservations about using self etch on areas of the plane that won’t see daylight. I’m only switching now for something that will hopefully be a little more resilient to high traffic areas.
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Primers are not designed as protective coatings. Their design assumes that a top coat will be applied for complete protection. Non two part primers can absorb and hold moisture. Epoxy primers, while also requiring a top, do provide very good protection from everything but UV and can be effective top coats for this application.

That self etch is better than nothing and does a decent job, just understand its limitations. I don’t think self etch works very well. Better to mechanically etch before application of traditional primer.
 
apivovarov said:
We are currently building an RV-12 and have been applying Rust-Oleum Self Etching Primer to all internal aluminum parts.

However, I’m not sure this is the best approach...

As I understand it, Self Etching Primer is not a protective coating on its own. It chemically etches the aluminum surface to promote adhesion, but by itself, it doesn’t offer long-term corrosion resistance. In fact, it removes the aluminum's natural oxide layer - which normally provides some protection - potentially making the metal more vulnerable to oxygen and moisture if left uncoated.

I’m now considering applying a topcoat over the primer - such as Rust-Oleum Automotive Gloss Acrylic Enamel - for added protection.

Can anyone comment on this? What would be the recommended approach here, without going overboard?
Click to expand...
Just clean, scuff and top coat.
 
Hi Lr172
Thanks for the helpful input. Could you elaborate a bit more on your suggestion: “Just clean, scuff and top coat”?
I have a couple of follow-up questions:
1. What kind of paint would you recommend for the top coat? Any specific examples or products that have worked well for you?
2. Is the Self Etching Primer (spray can) still necessary, or can the top coat be applied directly to the cleaned and scuffed aluminum?
Appreciate your insight - trying to keep it simple but effective.
 
I use sem ez coat as well for my internal priming, it has a bit of a gloss to it.
 
The thumb rule, “any primer is better than no primer”.

I prime every part before assembly with PPG DP-40LF. This is a pricey epoxy primer that is not self etching. Prep includes cleaning with Alumiprep and a maroon Scotch Brite pad (large pieces) or Colman fuel and Scotch Brite for small pieces and steel parts.

There are many options for two part epoxy primers, and I would guess all are a better option over rattle can stuff.

For interior surfaces that get seen, I eventually top coat with a single stage PPG urethane paint. Prep is a light scuffing with Scotch Brite and Colman fuel.

While much more time consuming, used for interior surfaces this primer does not need top coating and is amazingly durable. As a convenance I do use rattle can Rustolium primer for odds and ends (brackets and such) that never see the light of day and are not exposed to wear. While easy, you will be disappointed with the lack of durability of rattle can primers. While I don’t know, I suspect putting a top coat over these primers would add unnecessary weight, cost and most importantly incompatibility top coat paint options.

Carl
 
I wonder. There's a guy with the rattle can primed car in town. It's holding up fine after several years exposed to CO sun, road salt and whatever else. My car fender has a section over 20 years old. Granted not an aluminum airplane, but the inside of an airplane is far less severe environment. I'm not taking a side. Just wondering if we sometimes overthink this primer war.

I'm sure I'm wrong.
I'm the guy with a 2K epoxy interior and no top coat. Same primer has been on my 4x4 wheels for over 5 years, exposed to everything. Still look like the day I painted them.
 
apivovarov said:
We are currently building an RV-12 and have been applying Rust-Oleum Self Etching Primer to all internal aluminum parts.

However, I’m not sure this is the best approach...

As I understand it, Self Etching Primer is not a protective coating on its own. It chemically etches the aluminum surface to promote adhesion, but by itself, it doesn’t offer long-term corrosion resistance. In fact, it removes the aluminum's natural oxide layer - which normally provides some protection - potentially making the metal more vulnerable to oxygen and moisture if left uncoated.

I’m now considering applying a topcoat over the primer - such as Rust-Oleum Automotive Gloss Acrylic Enamel - for added protection.

Can anyone comment on this? What would be the recommended approach here, without going overboard?
Click to expand...
I used etching primer on the internal mating surfaces. I applied 2K epoxy primer in the cabin area. Everything else was left with etching primer or just bare alodine surface. On the opposite ends, the RV12 that was built as a part of the One-Week Wonder, doesn't have any primer in the interior surface. It is a part of the flying club fleet and made at least one long tour around the country. I have look inside 40+ years old Cessna wings and there are a lot of corrosion on bare metal surface. The old Cessnas don't have any primer treatment inside the wings. These Cessna were probably parked out in the open ramp, facing the elements for decades.
 
Sure - I use rattle can SEM on all the interior parts that are in contact with other interior parts (e.g. bulkhead rib flanges against skin, etc.)
 
wirejock said:
I wonder. There's a guy with the rattle can primed car in town. It's holding up fine after several years exposed to CO sun, road salt and whatever else. My car fender has a section over 20 years old. Granted not an aluminum airplane, but the inside of an airplane is far less severe environment. I'm not taking a side. Just wondering if we sometimes overthink this primer war.

I'm sure I'm wrong.
I'm the guy with a 2K epoxy interior and no top coat. Same primer has been on my 4x4 wheels for over 5 years, exposed to everything. Still look like the day I painted them.
Click to expand...
Another Colorado and NAPA 7220 self etch data point - YMMV, but 16 years ago I build Van's tool box kit to get my feet wet with the building/riveting/tool familiarization process prior to working on the real thing. I mechanically cleaned and prepped the metal and sprayed the whole thing with a couple of coats of 7220 and left it that way. After all that time it is still holding up well. Even after scraping it on garage floors for years. The only caution I would add is it will not repel oil stains and such, but as far as general durability is concerned, I have to say I never expected it to hold up as well as it has for as long as it has with no other top coat applied. Like Larry says, looks as good (almost) as the day I sprayed it.
 
Any organic coating is better than nothing. The enemy is dust and dirt that hold humidity/moisture against the metal and get the corrosion cell going. Alclad "should" be fine, but tell that to any bare metal plane in a hangar in my area. Nope. A mess of pitting in short order.
My go to is Akzo epoxy, very tough stuff. The vinyl wash primers are okay, but any decent solvent will take them off. The old VariPrime self-etching (sort of) were okay, but again, not an epoxy.
Use whatever is handy and you'll be better than nothing. If you need toughness, like in the cockpit areas, I would suggest an epoxy primer and urethane topcoat. Get some sort of etch on it, either mechanical or chemical to ensure it sticks well.
 
I'm with Carl, any primer is better than no primer.
When considering what is "best" consider where you live and how long you will likely own the airplane once it is built. In a maritime climate the something is required to avoid corrosion problems 10 years down the road. If you will probably keep the airplane only 5 years then rattle can etching primer is likely to be adequate.
For longer term protection use a 2 pack epoxy primer over scotchbrite scuffed metal and urethane/acrylic top coat. Epoxy primer is adequate in the parts you cannot see.
In an inland dry climate nothing to rattle can is probably adequate and will take a lot less time to apply.
 
Vans uses P60G on their QB’s.
When they do, they survive overseas shipment. The one time they didn’t, or used the wrong stuff, they corroded so badly they were not useable. I do not know if they sit at sea for days, weeks, or?
There are thousands of RV’s out there protected internally only by P60G. I have yet to hear of a single corrosion issue.
That said, even the P60G instructions state it “must” be top coated, but they are in the paint selling business….. ;)
 
I use Sherwin Williams P60G2 simply because I figured that “if it is good enough for Van’s QB kits it must be good enough for me”. Once in a while I will need one or two small parts primed and rather than mixing a sprayer full of P60G2 I have used both SEM self etching primer and NAPA 7220 self etching primer. I like the SEM product better than the NAPA but neither adheres anywhere nearly as well as the SW wash primer.

Advantages of P60G2: Easy to spray (Mix 2:1 instead of 1.5:1), weighs almost nothing, long track record, great adhesion, quick drying.
Disadvantages: need a spray gun and respirator, Not as bullet proof as an epoxy primer,
 
JonJay said:
Vans uses P60G on their QB’s.
When they do, they survive overseas shipment. The one time they didn’t, or used the wrong stuff, they corroded so badly they were not useable. I do not know if they sit at sea for days, weeks, or?
There are thousands of RV’s out there protected internally only by P60G. I have yet to hear of a single corrosion issue.
That said, even the P60G instructions state it “must” be top coated, but they are in the paint selling business….. ;)
Click to expand...
Shipments take many weeks by sea.
There are plenty of reports of corrosion of parts protected by only P60G2, particularly from builders in maritime climates.
 
apivovarov said:
We are currently building an RV-12 and have been applying Rust-Oleum Self Etching Primer to all internal aluminum parts.

However, I’m not sure this is the best approach...

As I understand it, Self Etching Primer is not a protective coating on its own. It chemically etches the aluminum surface to promote adhesion, but by itself, it doesn’t offer long-term corrosion resistance. In fact, it removes the aluminum's natural oxide layer - which normally provides some protection - potentially making the metal more vulnerable to oxygen and moisture if left uncoated.

I’m now considering applying a topcoat over the primer - such as Rust-Oleum Automotive Gloss Acrylic Enamel - for added protection.

Can anyone comment on this? What would be the recommended approach here, without going overboard?
Click to expand...
I made 4 long trays to immerse all parts in alodine. I staged as many parts as possible so I could treat as much as possible at once (alumaprep - rinse - alodine - rinse). I only had to do 2 such sessions for the whole project. Everything that could not fit in the trays was SEM EZ Coat. For the larger interior skin surfaces I didn’t see the point in removing that beautiful alclad coating. For that I fog ACF-50 once every couple years.
 

Sherwin Williams P60G2 longevity Test

Since it has been a while since any info about this test has been provided, I thought it was time for an update. As has been mentioned in the past, this test is about as un-controled as any can get and it in no way am I meaning to imply that it compares to any of the Mil Spec lab executed tests...
vansairforce.net
 
Lots of good comments here, but one piece missing. Ingredients.

Primers are either passive or active for aluminum corrosion. Passive is based on being a barrier, active means it has a sacrificial component. P60G2 uses
Pentazinc Chromate Octahydroxide. See zinc and Chromate there? Remember this.

The real question is not what primer is best but what is minimally needed to provide some protection for desired life. So what is desired life? 50 yrs? Inland or shore? Beware the rabbit hole. . . . .

Alodine is a surface conversion passivator and great adhesion promoter. The color looks like aviation too :-). There is a clear type but can't tell it is there.

So, no wrong way to look at primers, just decide how much protection is enough.

Personally, I prefer active primers. YMMV of course and that is OK, the RV will still last a long time. Enjoy the learning and decision making.
 
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