# Sealant between aluminum

Discussion in 'Sheet Metal' started by birdus, Sep 26, 2019.

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1. Oct 17, 2019

### BBerson

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Kenmore seaplane base uses non hardening 3M 5200 Marine sealant for saltwater floats.

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2. Oct 17, 2019

### pfarber

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Pressurized aircraft are just a *little* different than the bug smashers. Lets compare apples to apples?

3. Oct 17, 2019

### gtae07

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Anything that meets the milspec will work, regardless of brand. Last I recall, Van’s has the best deal going (a quart for the price most guys sell a pint). MIL-S-8802 is tank sealant and MIL-S-81733 is corrosion protection sealant, IIRC, but for our airplanes either will work for non-fuel applications.

Properly applied and riveted, the sealant has a negligible effect on static strength. It’s detrimental to fatigue but that’s typically accounted for in the allowables (more or less the specific strength values you’re using for given materials in your design).

4. Oct 17, 2019

### Richard Roller

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After doing many many repairs over the years I can say that all the major large airframe manufacturers call for sealant under the repair, plus edge sealing when installed. You are not putting a thick layer under the repair piece that would hold pieces apart. We always used a 4" paint roller to roll it on for a nice thin uniform layer. You still get some squeeze out when installed. While I no longer have access to the DC9/MD80 SRM, I seem to remember in the explanations that they considered the shear strength of the sealant as part of the repair.

Last edited: Oct 17, 2019
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5. Oct 17, 2019

### pfarber

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Again, this would be for pressurized AC? That are almost completely different than the GA planes more private pilots fly?

Paint works well. It will seal, unless your patch is garbage.

Reading the AC 43.13 I don't see anything about sealant for patch panels. Looking at page 4-21 shows common rivet defects, how can proseal NOT cause a few of them?

A patch panel should be the same metal thickness as the panel. I'd like to see you get a flush surface with sealant gooped between the metal.

6. Oct 17, 2019

### narfi

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(edit, its in 43-4, my bad)
Page 4-34, 4.13.6.4.10
As far as I am aware, this is common practice on small aircraft. I have sealed (almost) all small patches working primarily on small cessna's and pipers operated off of gravel (lots of patches) for most of my career. This has nothing to do with pressurization, though I am sure its even more important there. Paint cracks and peels, the same AC has pictures showing cracked paint allowing moisture to cause corrosion under and around fastners.

No one has suggested thick nasty applications of goo that would prevent you from setting a rivet properly, just a thin, even coating of sealant.

Lets be realistic here. We are talking about a small patch on what I assume is a relatively slow airplane (bonanza speeds or less) You could do zero work to the hole/crack and lay a strip or two of aluminum tape over it and paint it to match, it would last years.

If you don't want to seal the repair, its not going to kill anyone. But if you are going to do the work to make a patch (sounds like its not even a scab patch but the time put into an attractive flush patch) then why take short cuts that will end up with corrosion issues and/or moisture inside somewhere you don't want it?

Last edited: Oct 17, 2019
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7. Oct 18, 2019

### pfarber

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Sealants are useful in the following areas:
• Fuel tank;
• Pressure areas;
• Weather sealing;
• Firewalls;
• Electrical;
• Acid-resistant areas;
• Windows;
• High temperature applications; and
• Aerodynamic sealing.

I read this as nothing about structural repairs. The 43.13 is the bible or aircraft repair and does not mention inter-layer sealants. But since these are ACs (not ADs) you should follow the manufacturer, then the AC 43.13, then best practices.

I still say there is no need to apply a sealant unless you patch is garbage. Paint will more than do the job.

And I think this is more imprtant (you missed?)

Polysulfide, polythioether, and polyurethane sealing compounds consist of the base, or prepolymer, and the accelerator, or curing agent. When thoroughly mixed, the catalyst cures the base to a rubbery solid. Rates of cure depend on the type of base, catalyst, temperature, and humidity. A full cure may take as long as 7 days.

4.13.6.3.2 Silicone sealing compounds generally consist of one component which cures by reaction with moisture in the air. If silicones are applied too thickly or in such a way as to prevent moisture from entering the material, they may not cure at all. Many silicone sealing compounds also produce acetic acid, which has a vinegar smell, while curing, and can lead to severe corrosion problems. Limit silicone sealing compounds on aircraft to those noncorrosive products conforming to MIL-SPEC MIL-A-46146, Adhesives-Sealants, Silicone, RTV, Noncorrosive (For Use with Sensitive Metals and Equipment).

Good luck getting that flush surface.

8. Oct 18, 2019

### narfi

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**Not sure if you are serious or just playing Devil's advocate.
**To be clear on my position, I do NOT think your approach is the right one, however I do not care, I am for the most part playing the Devil's advocate.

(this is the section I point out how wrong you are)
We are not talking about silicone sealants, so your highlighting is irrelevant.

We are not talking about cure times, so your highlighting is irrelevant.

(this is where I redirect the conversation to the original purpose)
The First post was asking about sealing against rain, so while you do in fact quote the relevant portion, you failed to highlight it, or even recognize it.

(this is the part where I use your own words against you)
(this is the part where I beat my chest and say I am better than you)
I have done dozens, perhaps hundreds of these small patches. But all my work has been on small Certified aircraft, perhaps aluminum behaves differently on experimental aircraft?

(here is where I am a bit more serious)
There is nothing inherently wrong with riveting your skins together with wet paint, other than it is too thin, messy, toxic, and doesn't accomplish the intended goal as well as other options.

Options for a small hole poked through the side of your (relatively) slow fuselage:

• Ignore it - it probably isn't hurting anything
• monitor it - it may get bigger but is safe to fly for now
• hide it (its ugly)- cover it with aluminum tape because its ugly but isn't hurting anything
• hide it (its ugly) - stop drill the cracks and cover with aluminum tape because it isn't hurting anything
• scab patch - clean out the hole and rivet a chunk of aluminum over the top to keep the birds and squirrels out
• flush patch - you care enough to make it look nice and spend the time riveting a pretty flush patch that is invisible once painted over
I have done at least 4 of those......

For either of the patch types chosen you could,

• Install it dry/bare aluminum
• etch/alodine first then rivet on
• etch/alodine and spray with zinc then rivet on once cured
• install with wet zinc
• install with some cheap wet paint
• install with some wet polyurethane paint
• etch/alodine, spray with a light coat of zinc then once cured rivet together with a thin even coat of prc/proseal/flamemaster
I have done most of those as well, but only really consider one of them to be the 'right' way to do it.

**Intent with a grin is hard to convey over the internet.

9. Oct 18, 2019

### SVSUSteve

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It's still a valid comparison that you're not dealing with a "watertight" seal as you want to believe. How about you just accept that and we can move on?

10. Oct 18, 2019

### rick9mjn

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to add 2 cents...............on the forum ""vansairforce.net"" ,,there is a post about someone doing a pull test on a test strip that was riveted and glue- ed .......
...
..
. good day rick
imho-imbw,,,,,,i my humble opinion,i may be wrong

11. Oct 18, 2019

### BBerson

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Should you use sealant? Most Cessnas do not use any sealant or even paint in the skin joints faying surface. Only the factory seaplanes are primed inside. (older Cessnas, newer ones might have prime)

12. Oct 18, 2019

### Angusnofangus

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The older Cessnas that I have seen didn't have any primer inside, nor did they have sealed skin joints. The did, however, have corrosion not only on the inside, (especially in wings), but also corrosion in the joints. Cessna never built these with the idea that they would still be around these many years later.

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13. Oct 19, 2019

### BBerson

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They probably would corrode sometimes in Victoria, Canada and other salt water areas. I didn't see any faying corrosion in Alaska's interior. Paint or sealant can also sometimes lock in the corrosion. The periodic spray in fluids might be best, inspectable and doesn't lock in the festering corrosion. No simple answer.

14. Oct 19, 2019

### Angusnofangus

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I don't know where the Cessnas I observed came from, possibly the coast, but I was 300 miles from salt water at the time. But I do agree that there is no simple solution to preventing corrosion.

15. Oct 20, 2019

### karmarepair

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Good luck getting anything bonded with 5200 apart again! It sticks like grim death. 4200 is similar, with a quicker cure. CAREFUL heating to about 200 def F helps to release it, and there is a product call DeBond that actually works at softening it up enough to get it apart.

Moisture cure urethanes DO NOT produce Acetic Acid in their cure (which a previous poster has noted Silicone RTV DOES, to bad effect on aluminum). From a reference book on bonding electronics components:
Moisture-curing polyurethanes
Urethane polymer resins that still contain unreacted isocyanate groups react with atmospheric moisture and continue curing. The reaction is a two-step process. First, water reacts with some of the free isocyanate groups producing an amine and releasing carbon dioxide as a by-product; then, the amine further reacts with the remaining isocyanate groups forming urea linkages and fully cured polyurethane urea.
The Carbon Dioxide tends to make them foam, so they like tight joints. Gorilla Glue uses this same chemistry.

Home Depot has several other brands of moisture cure urethanes, by Loctite, PL, Sika, etc.

16. Oct 20, 2019

### BBerson

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I have seen corrosion under polyurethane paint also. The old zinc chromate paste was probably good for seams. Even ordinary black tar might work. There is $4 acrylic bathtub caulk, butyl rubber grey roof caulk is cheap ($2 for 8" tube)

17. Oct 20, 2019

### pfarber

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Its not about right or wrong, please grow up. Its a DISCUSSION about how to handle an issue. You seem to be taking this personally, if you didn't care, you wouldn't have killed so many electrons.

The 43.13 does not mention any need for any waterproofing on repairs. That's what I posted. Another AC related to corrosion prevention did talk about weatherproofing, while never specially mentioning repairs.

Its up to us, as educated and informed adults (well, mostly) to see what the best practice is.

My training/schooling taught me that repairs are made, then painted.

18. Oct 22, 2019

### litespeed

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No electrons were killed

They are just happy dancing along.

"Cessna did it that way" is not a relevant excuse for allowing corrosion to develop.

That excuse has caused many airframes to corrode and potentially fail.

"Ford built the Pinto" is never a accepted excuse.

You choose your outcomes, but don't then argue you followed best practice.

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