Self-etching Primer Compatabilty

[mustang]

Hi Guys and Gals,
I'm a new builder from Canada and I have my RV-8 empennage kit sitting on my workbench. I wanted to introduce myself and help to kick off this forum. I am quite familiar with this style of Forum as I frequent a couple of dirtbike boards similar to this one.

So far, I've just been practicing banging rivets into scrap aluminum before I get into any serious construction.

Today, I've just bought some "Self-etching Primer" from Lordco. I'm wondering how compatible this primer is, if I wanted to shoot acrylic enamel (catalyzed) over top of it. I'm talking interior structural parts here, not exterior.

Cheers,

[Dane]

I think that every RV builder goes through several different primers before setteling on one that they like. I started out with Zinc Cromate, gratuated to Vara-Prime (a self etching primer), and then went to a corrosion ressistant epoxy primer from Shirwin Williams. The bottom line, is that anything is better than nothing. Preping the metal before paint is probably just as important as the paint it's self - the process of etch and alodyne is time tested. If you use an epoxy primer and plan to top coat it with another paint, you need to apply the next coat within a few days, or scuff sand it - or nothing will stick to it. An epoxy primer will completely seal the metal and does not need to be top coated in areas you will not see (the insides of your flight controls, etc). Epoxy may be the best but it's also the most troublesom. It takes a long time to cure in a cold climate - I used to bring my parts up from the basement into the kitchen after the paint dried but before it really got hard.

[mustang]

Dane,

Thanks for all the info. If I was to use epoxy primer, (which I have), should I etch and alodine before shooting the epoxy???

My only experience with alodining was early this spring when I alodined a Citabria spinner before priming and painting with acrylic enamel. It turned out pretty good. Is the etching process essential before the alodine???

Thanks, Pete

[g_e_young]

There some good info at http://www.rv-8.com/Home.htm about paint and primer. Randy recommends SEM self etching primer (available in a spray can). I've haven't used it yet, but I build an RV-6 using SW self etching primer which required mixing and it was a pain in the rear.

None of us will live long enough to experience the effects of the various primer technologies on our airplanes, so I believe in using what is simple and quick.

grant-

[Guest]

Hi all,
For the interior surfaces I just started using a self-etching primer from PPG. It is SX1031. It is nice in that there is nothing to mix. I just pour it in my $50 HVLP Harbor Freight special and spray away.
I first clean the parts with acetone. Then I clean with them with another PPG product designed to remove grease and wax. (I can't remember the part number right now, but it is the one recommended on the SX1031 data sheet). Next I scuff the surface of the parts with a gray Scotchbrite pad and then clean with the grease remover again. I then shoot the parts usually within an hour. I have waited overnight before and the parts seemed to do fine.
I would appreciate any comments pro or con by those of you that know about primers and paints. This is not my area of expertise.

Tony

[OutlawRV8]

I started with the NAPA 7220 rattle cans on my emp. and now I have switched to the AKZO sold by Aircraft Spruce. Its one to one mix and dries fast. Not cheap but I think the gallon supply will do the entire airplane, Im not doing the inside of the skins but I am doing the alclad parts in the skeleton. When I first started researching primer I just got tired of surfing all the various sites with primer opinions and settled on one and got on with it. You can eat up a lot of shop time doing things like this. I built a homebuilt in the early '80s and did just fine without the innernet, but Im glad I have it today, kind of like cell phones. I recently sold a '55 Cessna 180 that had no interior primer and was clean as can be and this airplane had spent a few years on the east coast early in its life, I bought it in '84 and had no corrosion showing anywhere. I live in the midwest and I know of an RV built in '86 with no interior primer and it looks brand new inside. Kevin

[Spike]

Im a fan of just spraying the parts where they contact other parts or have been scuffed up. The rest of it Ill just leave bare metal. My '73 172 is nekid on the inside and looks just as pretty as can be.

-- John

[captain_john]

Well, it is official. Our first primer war!



Not a war really. Everyone is very civil, so I am comfortable weighing in.

I am using DuPont Vari-Prime.



I understand it isn't a "sealing primer" meaning the pigmentation isn't as dense as perhaps some others. The "real" job of any true anti-corrosion measure is to place a "durable barrier" between the substrate and the environment.

Let me state that again. The "real" job of any true anti-corrosion measure is to place a "durable barrier" between the substrate and the environment.

Now enter the definition of "durable barrier". Is a coating of wax considered durable? Well, it is for a limited amount of time. A film of motor oil would do the trick too, but that isn't feasible as it collects dust and gets on your clothes. Now paint, that would do the trick! That is where we have all our choices.

Something, is better than noting and applying that "something" in an approved (perhaps in an ASTM standardized fashion) and safe fashion is best.

It really comes down to your exposure to environment, availablility and ease of use. That is why I chose DuPont for now. It may change in the future.

CJ

[g_e_young]

So can I use Zinc Chromate without etching first, or is this why people use self etching primer?

http://www.aircraftspruce.com/catalog/cspages/zinc.php

There's something nice about using something that's been used for over 50 years!

grant-

[Dave Oakes]

My question is this: before riveting parts together should a person prime the interior of the skin and then rivet the parts? Then to go one step further, should one put a finish coat and then rivet the parts together?


thanks for you ideas.
Dave

[mustang]

Dave,
I have a friend who is building his 6th RV. (I think he likes them!) He doesn't use any primer. He only coats the non ALCLAD parts like the horiz stab doublers with epoxy primer. You know, the stuff made of 6061-T6. He is a firm believer in keeping it light and having a fast performance airplane. He says to me all the time, "We will never see corrosion in our lifetimes on these airframes". Since we are in our 60s, I have to agree with him. I'm thinking of shooting (misting) a little bit of primer where the ribs and spars are riveted to the skins, just on the inside of course. I'm working on finding a modern, zinc rich primer to preclude corrosion from the dissimilar metals in the presence of water. I'm thinking of something a little bit more modern than zinc chromate from a spraycan here. An epoxy primer with no thinners or reducer would work, as after setting up, it is all solid material and there is very little evaporative in the composition.

I would not finishcoat the interior control surfaces, nor would I then rivet them. Paint shrinks for some time. The last thing you want is a thick, shrinking layer in between your spars and skin. That would seem to be a recipe for loosening rivets in my mind. That is why they tell you to never paint the backing plate for a prop, or, the back of the prop before installing it. The torque on the bolts will slack off before long and you will have to retorque the bolts and relockwire. It might be harder to retorque all your rivets though!
Cheers, Pete

[captain_john]

Well, the need for Zinc Chromate has been eliminated by using the self etching primer. That doesn't mean you can't do it if you want to. Likewise goes for Alodine.

Technology has come a long way since those products were used. These new S-E primers have "teeth" and in addition to your "profiling" the AL before application of the product and the products "bite" you should not suffer detatchment provided the material has been prepped properly (clean and dry) and a quality product is used.

What to paint... everything once. If you cut a smashed sample rivet in half, exposing a cross-section you will see the rivet and sheets virtually become one. This will also include the paint. This paint is supposed to be no thicker than 1.2 mils thick. After it is sandwiched between sheets and riveted it may become .6 mils thick or less. Even if it does shrink, this will be a negligible factor. A loose rivet is the product of a well used (or abused) machine.

Anyhow, paint all components that overlap and come into contact with one another. Also paint any scratches or edges (scuff 'em first with scotchbrite). For me, this means paint the entire inside of the plane. I live in New England and am planning to fly this plane into my retirement. I am 38 now and reasonably healthy. Let's hope I stay that way. (healthy, not 38)

CJ

[captain_john]

BTW, a dollar bill is 3 mils thick. So there is a reference for comparison.

CJ

[jim_geo]

Is there a significant difference between an anticorrosive and a primer? Am I right in thinking that a primer will suffice as an anticorrosive? Is the environmental barrier created by primer enough to protect aluminum or is weight the only real issue? As I understand the function of primer. Primer is used to adhere to a surface being finished in this case primarily aluminum, While also adhering to the material being applied (paint) and primer becomes the bonding layer.

[captain_john]

Yup, yup, yup... you are on the right track.

I am assuming you are comparing priming to alodine or other chemical applications. Alodine is a "conversion coating". The difference is when you alodine a part the surface layer actually oxidizes. Essentially it corrodes the finish. This is similar to the blueing process on a firearm. Once these processes are performed to the material, it mitigates any further corrosion.

Alclad is nothing but a pure layer of aluminum applied to the surface of the aluminum sheet. The 2024 we use is an alloy. It has other stuff in it for strength and formability. The "alcladding" is designed to inhibit corrsion due to it's natural properties.

I chose primer for ease of application and material handling.

I hope this helps.

CJ