Thorp Air Command - T18.net

Some elementary questions.

Just thinking about long term proper corrosion protection.

Is etching and alodine enough?

What type of primer is best? Epoxy primer?

Is it best to prime the entire inside or just where two peices contact each other?

Do you have to prime after dimpling and prior to riveting in order to protect the hole and between the sheet and rivet?

I see it like insurance, you can definitely buy too much! I use a light dusting of zinc chromate on all parts. Yes I focus on faying ( mating) surfaces.
Cubes

Dick Van G. Says prime the inside if you want your great grandkids to fly to plane. Otherwise, geter' done. I did what Cubes said and have no interior corrosion after thirty years.

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Bob Highley
N711SH
SN 835
KLAL

I pulled a roll of 2024 from my dad's attic, 1/4 mile from Atlantic ocean,. After 35+ years with absolutely no protection there was only minor pitting on exterior of roll only. Interior looked fine & I still use it for parts today.

OK.

So what type of primer is best?

I have seen aviation primer in aerosol cans. Is that sufficient for a quick spray where two members come together?

Or is it best to get an air brush and have to mix up two part epoxy primer like stitts or whatever name they call themselves now?

Or some other product?

I used zinc chromate spray in a rattle can that I got at an auto parts store for about $6.00 a can . Perfect solution when assembling small sections , like the rudder , flaps , ailerons , etc. Not sure what Spruce sells it for , but its got to be close to $10.00/can ? Not sure if you are going to get an epoxy primer to flow thru an air brush ?

Any good primer that is specified for aircraft use will work . I found the spray cans were an easier option for the small jobs rather than mixing up , thinning up , cupping up and dragging the hose out to hook up to the gun . There was also almost NO clean up involved . BOOM !

Before Zinc Chromate 595 Spec can be applied, an etching material must be used, such as alodine and is not an easy under coating to apply. Tack free in six hours, dryes in ten and cures in seven days. Conditions (ambient air) must be so, so and must be top coated within a window or a light oxidation will form which can lead to top coat adhession degradation.

I prefer Dupont VeriPrime which cost around $400 a gallon by the time you get the activator which actually gives about two gallons. Nason self etching primer on the other hand is about $150 a gallon with activator, Chrome free and does a good a job. Nason is a lower grade Dupont product and many auto repair shops use it. It's much user flriendly, easier to prepare for and apply. Can be sanded within thirty minutes and sealer must be applied as with any other primer product. Can be top coat the same day.

By using Self Etching Primer you eliminate the use of Alodine to etch the aluminum. Alodine and Self Etch Primer do not like each other. If you use Alodine, do not use Self Etching Primer. Alodine must also be applied a certain way and light scotchbright rubbed, tack rag prior to zinc chromate or other primer and may allow top coat to lift if not properly applied; a process that can eat up a full day or two even before primer can be applied.

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In CowTown USA,
Don D-Day
NX18DD

I admit that I went way overboard when building my Thorp. I constructed a small tank & a variable DC power supply to chromic acid anodize anything that was small enough to fit in my tank. I Alodined everything else inside & out right before final riveting, and coated all faying surfaces with a light coat of mil spec zinc chromate, usually from a rattle can, but sometimes with a gun. I then coated most all of the internal surfaces with a light coat also. I've read that it only takes a very thin see-through coat to protect the aluminum. It was available in rattle cans at many paint stores in those days, but is considered a hazardous material now days. As Rich said, it's still available from Spruce in rattle cans. It's probably still available to professionals in bulk also. Some guys preferred the yellow stuff. I always liked the green. Definitely don't use Zinc Phosphate in my opinion - I don't think that adhesion is as good for one thing. I've also read that it's the chromate ions that protect the aluminum. Epoxy based primers are very good, & I used that for my fiberglass pieces, but good old zinc chromate was my choice with the aluminum for simple, easy application & fast drying. I too like DuPont VariPrime, & I used it a lot under my top coats of PPG Durathane. My experience is that the VariPrime works well over an Alodined surface. I still have samples of alodined, then VariPrimed aluminum that I have tried to remove the primer from. The adhesion is exceptionally strong, & it's difficult to remove with anything other than a good paint remover. In 24 years I've never had any kind of paint failure on my bird (cracking, blistering or peeling), and all of the exterior skins were Alodined, primed with VariPrime, & top-coated with Durathane. That's just my experience. I tended & preferred to use a red rust-proofing primer on my steel parts, but sometimes zinc-chromated them.
I wouldn't go to quite the same amount of trouble if I was building again.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

There are two answers here to two different questions. My answer was on how to protect the inside of the plane before assembly. I have no experience with prepping the exterior for paint.
Cubes

On the exterior I used a 2 part high build ISO free epoxy primer from TCP , Tri City paint .

Rattle can of self etching primer sounds great to me at least for the mating surfaces while building. However for whatever reason most of my rattle cans of paint quit working about half way through the can if I use them infrequenntly over time.

I suspect that if I build I will start with something like the rudder and/or vert stabilizer flaps, ailerons etc. And would like to have some protection before closing it up.

All primers seem to indicate that they need a surface finish as do many of the comments above. So if you don't paint over the primer is it still protective? I don't know anyone that I've ever seen or read about that actually covers the primer for interior surfaces.

Exterior aircraft overall finish would not seem to be a place to be cheap on in relation to prime coat.

If you actually painting a part and buy the rattle can of self etching primer from "Checker Air Parts", or "Air Zone" there is a "sandable" version - I cannot remember which one is or isn't, but it does say on the can whether it is sandable.

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Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

SEM makes a self etching primer in a rattle can that says "for steel & aluminum". It's sold in automotive paint stores. I've seen RV builders in this area using it. I got a can (it's pretty expensive), & tried it with my current project. It seems to adhere well to steel, but I'm not impressed with it's adhesion to aluminum. Also, I'm not sure how good it actually is between dissimilar metals. That's where zinc chromate really excels, protecting the aluminum from galvanic corrosion between it & steel for instance.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

is there a provider of a plain zinc chromate in a rattle can?

If so I assume I'd still have to Alodine before spraying the zinc chromate right?

Acid etch and Alodine first. Then primer. I believe zinc chromate is off the market due to heavy metal concerns. If you do find it, make sure you use a good mask when applying it. There are self-etching primers out there, but I haven't had much luck on aluminum with it.

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Bob Highley
N711SH
SN 835
KLAL