Sealant between aluminum

I need to create a patch with a filler piece riveted to a backer with the backer riveted to the fuselage. The material will be .032" 2024. Should I use some kind of sealant between the pieces, or will the (flush) rivets prevent the ingress of rain?

Thanks,
Jay

 

"Pucky" you need to put some pucky on it

How big is the patch?
Its probably a good idea to use something. For small patches prc or even jb weld (or qwik weld) works well. For larger peices there is a fuel resistant coating I use but forget the name of. It is really thin but when left in the can becomes thicker, I've seen it in honey color and red. If I remember I'll try to find it when I'm at work today.

 

It's about a 2-inch hole. I imagine there'll be an inch of overlap between the backer and the fuselage.

 

In that case, whatever pucky you have laying around should be just fine. Thinner is nice, you don't want a super thick stuff keeping the patch from laying flush. Fuel resistant is good, you don't want random chemicals loosening it up and running out down the road sometime. JB weld is cheap and easy to find in your local stores or laying around your shop. It is a little thick, but just work it down thin before sticking it together and there isnt any problem with it.

I didn't check what it is we have at work, but it is this, or similar to it,
https://www.3m.com/3M/en_US/company...t-Coating-EC-776/?N=5002385+3292668364&rt=rud

It is nice on bigger patches or skin seams and keeps oil, fuel, etc... from working in between the skins and working the rivets loose. I just brush it on the seam with a chip brush before clecoing and riveting them together.

 

I think the word you are looking for is 'paint'.

If you do a good job and have tight fitting patches then primer/paint is enough to stop even capillary action. But if your workmanship is not that great or you have large gaps/seams then look up 'proseal'. Its sealant that's used for everything (even structural components).

 

I have to disagree, primer/paint will most definitely NOT keep water from getting between a patch and skin. Once you get water between layers it will eventually corrode, no matter how good your primer and paint are. The only way to keep water out is by using a sealant, PR 870 is a good all-around one.

 

A patch that is riveted in place and painted is not going to move. We are not talking an inspection panel that has to come off, but a permanent repair. Paint will will voids and stop capillary action. Now if your patch sucks and had significant gaps, then no (just as I stated).

I'd like to see the patch repair that properly done and painted that allows water to pass. If it does, its simply not done right.

 

Do good workmanship, deburr everything, spray a thick coat of Zinc Chromate or Zinc Phosphate spray on the parts, and put it together "wet". then another thick coat of this zinc spray after it's done, and you should be as good as whatever Cessna and Piper and Beech built. If you want to alodine the parts that's fine too. but alodine will not stop water from getting between gaps.

 

There was a China Airlines flight (Dynasty 611 if memory serves) where they placed a patch (admittedly far too small to account for cyclic loading during flights) after a tail strike. One of the ways the issue was identified after the in-flight breakup was that tobacco smoke and tar has leaked around the edges and stained the fuselage. Basically, that riveted and painted patch was acting as an inadvertent pressure relief valve.

A riveted and painted joint may originally be water tight in and of it's own right but with the nature of aircraft operations, it is not as certain to remain that way as you seem to believe.

Water, to use the words of Ian Malcolm in Jurassic Park, "finds a way". That's why so few things on aircraft should be treated as "if it was done right the first time" when it comes to maintenance or design.

 

Ever seen smoking rivets?
Ever seen them after someone hosed the inside out with CorrosionX and flew it for a few hundred hours?

 

Obviously a patch that is riveted in place is not going to move. The only thing that will stop water ingress is sealant, not paint. I've put hundreds, if not thousands, of patches on all sorts of airplanes and helicopters, and one thing they all had in common was they were put on with sealant. Paint will not keep water out. I've never seen a manufacturers SRM that didn't call for sealant under repair patches.

 

Sealant under the patch or edge sealed. Under the patch can and will screw with ones design allowables for installed rivet strength.

 

I'm not an engineer and know nothing about 'design allowables'. However, I do know that sealant under repair patches is the industry standard, as referenced by every manufacturers Structural Repair Manual that I have ever seen.

 

Over the first 20 years of C-130 production/service, Lockheed learned a lot about corrosion protection. Metal parts should be primed and faying surfaces sealed, preferably with a corrosion inhibiting sealant that contains dichromate. Fasteners are coated with sealant before being installed. This is all messy and you'll wind up with sealant all over your clothes. You want as much chromate as possible so ideally, you would have Alodine before priming with zinc chromate or chromated epoxy primer. One of the production test pilots said that the C-130 was the darndest airplane he'd ever seen because you couldn't stop water from leaking in the top and couldn't make it drain out of the bottom. When we had three major airframers, the opinion at Lockheed was that Lockheed had the best corrosion protection, Douglas was second best and Boeing was worst. Today Boeing has some applications where they apply Corrosion Preventive Compound to faying surfaces.

 

I would bet that even though they might not be fay sealed, I'll bet they are edge sealed in an effort to keep water out.

 

check out,"pro seal" aircraft spruce carrys it. it is a two part sealant( even sets in middle of patch) it is used as gasket sealant and for sealing off sections of a wing to make a ,"wet wing". that is a wing section to hold fuel.
they used to market it as a marine sealant under the name"paraseal" but it is no longer in marine outlets. it is called a poly sulphide. distinct smell of dogshit when the can is opened. you won't need gaskets for anything if you have this stuff! helicopter mrchamnics say it is the only thing that keeps turbine oil inside the cases it even keeps oil in british m,otorcycle.. yesss! it is that good!

 

If the patch is needed because of cracking, sealant will help it not crack again. Pressurized planes have to have patches with sealant for many reasons. Corrosion, pressure leaks, dampening for cracks or the added bonding strength. I always do it because my first stop in aviation was airliners, and it has not failed me. Structural patches. If you are just trying to fill a non structural something, I have put some bugger patch on just to say I did and move on. One size does not fit all.

 

"ProSeal" Remember it well. And have the clothing stains to prove it. Used to call it "Fort Rucker Taffy" because it was slathered every where. Guess it kept most the water out, but was a bugger to get off when parts needed to be removed for inspection. Hope you can locate the Right Stuff.

 

FlameMaster is the other brand

 

Pro Seal and FlameMaster are two brands of two part polysulfide. PRC-Desoto and 3M also make it, in a dizzying variety of forms, thin, thick, frozen patties (I'm NOT kidding). Good stuff, but a little hard to find in the boonies, or overseas, in retail quantities. MIL-PRF-81733D is the governing standard. Invented in the late 1930's, and the standard in military aviation until today, for aluminum structures.
Zinc chromate PASTE, sort of like Permatex #2 with added carcinegins, is extinct, see the movie "Erin Brockovich" for the reasons why. Zinc BORATE seems to the substitute, and the green spray can primer used nowadays is THAT. Duralac is one jointing compound PASTE you can actually, like, buy, but I'm not clear how, as it's active ingredient is Barium Chromate, two "EPA Metals" in one. https://www.fisheriessupply.com/saddington-consultants-plus-duralac-anti-corrosion-jointing-compound 'spensive.
The Airstream trailer restoration people use either moisture cure polyurethanes, or "Gutterseal". You can do the same, PL Premium is available everywhere, and there are similar 3M products. https://www.vintagetrailersupply.com/exterior-caulks-sealant-s/24.htm You think polysulfide is hard to get apart, you ain't never tried to get cured urethane apart.

Heat is the secret to getting any of this to release if you have to disassemble it, but as little of it as you can get away with, to avoid messing with the temper of the aluminum. GENTLY play a propane torch and use color sticks to keep from getting it too hot.