Priming alum sheet as a finish

[Zzz]

As I work on my boot cowl, panel, glare shield, etc I've also started thinking about how to finish the aluminum surfaces that will never see the light of day. For instance, the back side of the panel, underneath the glareshield, under side of the floorboards, inside the engine cowling, etc. Am I right to expect that you guys are not top coating these places to save weight and effort?

In my studies I end up reading a lot of thread on VansAirforce, and see that many of those guys are priming pieces before riveting. Those QB kits actually come from the factory with a metal wash prime (Sherwin Williams P60 I think) for corrosion protection, as it's a minimal thickness coating. Correct me if that's not accurate.

I've seen first hand the effects of moist environments on Alclad joints in my experience with old Cessnas, and I just want to protect my Bearhawk properly. These are things that let me sleep soundly, but can also factor into resale value. Regardless whether you think corrosion is a risk, are you finishing unseen alum either with just primer or full top coat?

Thoughts?

[Battson]

With Alclad sheets I can access easily (i.e. cowl and boot cowl), I chose not to finish them with any paint / primer. I am prepared to wait and deal with corrosion, if it occurs, but I am optimistic. Once the alclad has passivated, it should act like a primer.

The aluminium corrosion issues I have already seen are certainly not those places I decided not to paint. Wear seems to be the cause.
The wings and all the doors are already smoking or bubbling in a few places, where:

a) tank bay covers (which sit raised above the wing skin)
Where water gets into the tank bay cover, only during flight in the rain. I see very small bubbling under the paint, around the stainless steel screws (galvanic corrosion). Note these sheets were painted on both sides, didn't help at all. I have now sealed the leading edge of the tank bay covers, with an RTV gasket. This has stopped the process

b) the wing skins overlap.
These areas were also painted on all sides to start with, but I can only assume the strains associated with flight loading have caused enough movement to wear at the paint down - at the leading edge only. This is where there are no rivets, and the fit is the hardest to get right. I need to find a way of filling these areas to keep water out, before the passivation layer is consumed. I am thinking of epoxy and a fine needle. I already used a very malleable paint, didn't help.

c) doors press against the body.
All these areas need to have thin tapes / seals applied prior to first flight. Cowl doors and the people doors too. The paint wears down in a few hours of vibration. Much easier to prevent this problem than try to fix it later, once the smoking has started. The corrosive process prevents tapes from sticking long-term.

So, where the panels aren't touching and wearing, the alclad does a fine job (so far). Wear seems to precipitate the most corrosion at this early stage, but we do hanger within 20NM of the sea.

[Zzz]

Good point, Jon. Wear-related corrosion is a big deal, because even if something is primed, top coated, whatever, wear from metal on metal through to the alloy will be rapid.

[Mark Goldberg]

Zane, kind of "standard practice" is to primer aluminum that is covered up. Like inside your wing or the insides of the boot cowl. The aluminum sheets you got with your kit (floor/boot cowl/cowling) are not primered with anything. Just alclad sheets. I have usually primered them lightly in the spots you mention. Zero reason to top coat with heavy paint where it won't be seen. Mark

[marcusofcotton]

I see very small bubbling under the paint, around the stainless steel screws (galvanic corrosion).

This, and galling, are the reasons I tend to shy away from stainless steel screws. Cadmium or Zinc plated screws are generally cheap enough to replace as the protection deteriorates, much easier than replacing aluminum structure damaged by the stainless steel.

[Battson]

The trick is not to let water (especially salty water) get in there / stay in there for long, so there's no electrolytic action. That was my problem. Kept dry, the potential is low enough to be acceptable.

Galvanic corrosion between Al and Cd / Zn can happen IF you have electrolyte. More so with Zn. But of course, the Zn is the sacrificial metal with pure Al - but with a 20 series Al alloy - like we have in aircraft metals, less alclad, the Zn / Cd will consume the Al-20 series.

So it's still a problem with Zn / Cd, IF you get an electrolyte in there. Albeit a very slow acting problem.

[LukeS]

Anyone know how floatplanes successfully fight corrosion from salt water exposure? Is it just making sure to wash the plane after each outing?

[Battson]

I don't know about any special tricks people might be using.

Ever looked inside the body of a 40 year old sea plane? Even paint can't stop them going rotten :'-(

With some of the amphibs / float planes I have seen, they wash them in fresh water every night, or at least keep them moored in fresh water whenever possible.

Whenever we do beach landings, I always hose the Bearhawk off afterwards.

[LukeS]

I may have to seriously reconsider how much I want to mount the Bearhawk on floats in the future. I couldn't bear watching my pride and joy rust away. It may just have to stay on Bushwheels.

[N942VT]

Epoxy primer is good without a topcoat in those areas. A light coat is all that's needed and is good practice.

[Bcone1381]

Anyone use CorosionX?

http://www.avweb.com/news/maint/aluminum_corrosion_treatment_aircraft_aviation_acf50_208138-1.html

[mswain]

The Bearhawk builder I know locally treated the inside of his wings with CorrosionX after his build was complete. He built some long wands to run down the lightening holes.

[dedgemon]

For reference, the primer used by Vans is a Sherwin Williams product, P60G2. I expect that being on the left coast that they are using the version without the zinc chromate content. They are spraying a clear version, the P60G2 I've used is green and contains some zinc chromate. I've used the real stuff on three airplanes and like it very much. Its a compromise (what isn't?). This is a wash primer not an epoxy. It is NOT as good as the epoxy, but its also MUCH lighter, MUCH simpler to use (almost no prep), MUCH cheaper to buy. And it does in fact add significant corrosion protection. My RV9A is 12 years old and the inside of the airplane looks NEW. Note, that it is hangar stored but it still has all of the moisture issues that come from sitting inside of a uninsulated tin can!

I personally think that this is a very acceptable compromise priming system for the interior alclad. Of course I don't live seaside either!

[Zzz]

The Bearhawk builder I know locally treated the inside of his wings with CorrosionX after his build was complete. He built some long wands to run down the lightening holes.

Ugh.

I've applied Boeshield and Corrosion X on Cessnas, using a fogging system, and I would never do it on an airplane I built new. The reason for this in the first place was that the insides of those Cessna wings were not primed at all unless they were seaplane config from the factory, then they got a zinc chromate primer. Many of those old 50's and 60's Cessna wings suffered surface corrosion over the decades inside where there was no paint or primer at all, and an as a method of stopping the corrosion reaction from continuing and spreading, products like Corrosion X, LPS, or Boeshield were fogged in there. It makes for kind of a nasty mess, and future painting is complicated.

If you have the luxury of building your own aircraft, just prime the insides of the wings and there won't be a reason to use a corrosion inhibitor down the road. Much less messy.

[whee]

I'm in agreement with Z. Corrosion X isn't any fun and I can see no reason to use it on a new build. At this moment my plan is the same as what Battson did but I live in a very dry climate. We used rattle can zinc chromate on our Luscombe rebuild with no top coat. It was doing great on the aluminum parts after 10 years.

[Zzz]

Epoxy primer is good without a topcoat in those areas. A light coat is all that's needed and is good practice.

Which brand/product do you use, Dave?

[N3UW]

I am not sure what Dave uses but I use an Strontim Cromate Epoxy aerospace primer. It has been a great primer. Easy to spray and rock hard. Nothing bothers it. I get it from Spruce. Comes in 1 gal base and 2 gal cat. About $200 for the 2 gal kit. Highly recommend it.

https://www.aircraftspruce.com/catalog/cspages/akzoprimer.php

[Zzz]

For reference, the primer used by Vans is a Sherwin Williams product, P60G2. I expect that being on the left coast that they are using the version without the zinc chromate content. They are spraying a clear version, the P60G2 I've used is green and contains some zinc chromate. I've used the real stuff on three airplanes and like it very much.

I think a guy would have to get the version with the zinc chromate. I'm not sure why it wouldn't be available in Oregon. I can go down to West Marine and buy ZnCrO4 in the rattle can.

[dedgemon]

Just a guess on my part since their primer is clear. i guess they may be using something other than the P60g2 but that seems to always be what is discussed. The product I buy from SW is always a nice green color and the MSDS clearly lists the zinc.

[N942VT]

Poly Fiber's EP420 epoxy is really good. I like PPG DPLF epoxy as well. I've use Stewarts waterbourn epoxy, great on aluminum, not so much on steel. For rattle can touch ups and small areas I like SEM Products self etching primer. Always alodine, then prime for aluminum. Light coats, not too much, one tack coat for adhesion and one good coat to cover.
Zane, still trying to get in touch on the engine discussion, I've been working out of cell range for weeks now but try me again. Dang cell coverage around here.