Fabric separation - belly and wing root

B

Bob Turner

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Most of the higher power ACA products get beat up pretty good in the propwash area, notably the belly and the fuel tank areas on both wings.

I just had a 7GCAA 160 with gorgeous urethane finish paint separate on the right side underneath at the second double rib. Looking under the separated tape, it looked like the mechanic had used blue tinted dope.

Realizing that repairs must be done using the original materials, I took the liberty of engineering a minor alteration - using some card stock I flooded the area with Poly Brush, waited until tacky, then pressed the tape back in place. Next day I put some heat on it hoping to seal it.

Looks ok, except you can now see the tape edges. Made it through its first hour of pattern work - so far, so good.

I should have taken before and after photos - just picture about two feet of surface tape hanging down like a sling or belt.

So - why do you think it failed? It was part of an $11,000 repair job done last year (I had volunteered to do it for free, but I do not spray urethanes). Could it be not enough nitrate dope before applying tapes? Or possibly the urethane finish weakening the dope?

Expecting more such failures. Owner is a bit dismayed, considering the investment.
 
I would guess poor technique of some kind. Insufficient dope to wet the tape, did not pre-dope the tapes, let the dope start to dry before applying the tape, temperature or humidity too high, etc.

Certified Coatings nitrate dope is blue tint.
 
Did not know Certified was still in business. That was my guess, too, but I have not had tapes pull out like this before. I don't really remember doing anything special when using nitrate/butyrate. I am starting to put extra layers of Poly Brush on tapes - hope I am not over-doing it.
 
Big Ed said:
I would guess poor technique of some kind. Insufficient dope to wet the tape, did not pre-dope the tapes, let the dope start to dry before applying the tape, temperature or humidity too high, etc.

Certified Coatings nitrate dope is blue tint.
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with dope, it is the adhesive and the paint, right?
 
Bartman said:
with dope, it is the adhesive and the paint, right?
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I can only speak to the Ceconite STC, which AFAIK is the only all-dope process. No idea what various urethane topcoat processes require.

The STC adhesive for anything structural is Super Seam II.

Reinforcing tapes are applied with nitrate dope. Then all the fabric is coated with nitrate dope.

The exterior coatings are butyrate dope. That includes layers of clear, then silver, then color.

The base coat of nitrate is required because butyrate does not adhere to dacron.

I repaired my loose tapes with Super Seam, and also used it for reinforcing tapes in problem areas where the base fabric was weak. It is considerably stronger than nitrate. Pulling off a tape glued with it is a challenge.
 
Thanks for the explanation Ed! Super Seam to stick stuff to the frame or wings, nitrate for attaching tapes and base coat then butyrate dope for all additional steps/layers.
 
I probably should have stuck Super Seam in there. I suspect they just didn't use enough nitrate. My personal Decathlon was done with Ceconite (nitrate/butyrate) anf finish is polished butyrate. It does have a few lesions in the propwash area, but nothing like the above tape separation. And it is 30 years old with a thousand hours on the fabric.

I am probably going to check it twice a week to make sure the Poly Brush holds. It flies two-three hours a day.
 
Rumor sez that Super Seam II and Poly Tak are the same stuff with a different label on the can. They are made and sold by the same company so I would not be surprised.

Super Seam II is much stronger than the original Super Seam. Hence the STC change from old school sewn-in patches to 2" overlap glued-on patches.

One very important distinction between the 2 processes. PolyFiber allows repairs glued onto the exterior coating, while Ceconite requires stripping to bare fabric. That is not because of differences in the cement.
It is because adhesion of either cement to BUTYRATE dope is poor. I did some test swatches and confirmed this.

Both cements are nitrocellulose-based, as is NITRATE dope, so I believe that bond is quite strong. Based on that, Bob, I suspect your repair is going to be bulletproof.

The Stewart rep told me their cement is STC approved for repairs to ALL processes.
 
I have used Stewart cement as well. I like it for some applications, and I get smoother patch repair with wet-sanded Eko-Fill. But our trainer Cub has had tape problems (I didn't cover that one either) and my only truly successful repairs have been with the Poly Brush/heat seal technique.

Might be a good thing I went that way here. It is possible they put the tapes on with butyrate.
 
If it was blue tint on both the fabric and the underside of the tape, it was nitrate. Most likely they just didn't use enough, or let it dry too much.

I don't know if nitrate dope blushes like butyrate. If so, that could be a cause too. I had a few spots of blushing on one of my silver coats, and it created a very weak bond. When I pulled off the N Number masking tape, it peeled the dope off down to the blushed layer. The weakness of that layer was obvious from the appearance. Very granular and rough, not smooth.
 
Ed,
No part of Polyfiber's process contains any Butyrate or Nitrate dope. Polyfiber is all vinyl components.
And while you CAN use Poly-tac to glue a patch on top of Polytone (single stage) it makes a less than stellar bond and a crappy looking repair. With the Aerothane (catalyzed urethane) you need to sand off down to silver at least. Better repair is done with both by going to bare fabric.
Chris
 
Semi-related thread hijack question: I have some old repairs to loose tape edges that look like they were made with translucent yellow cement. Any idea what that is?
 
Nope. But I am not going to admit to having patched over Butyrate without going to silver or fabric. Not admitting to doing it in the slipstream on my Decathlon, even though if I did it, it was 17 years ago. But if I had done it, I probably could have reported that you can hardly see it, and that it never pulled off. Certainly couldn't admit that multiple repairs for balun antenna and changed N-numbers were involved, let alone heavy duty belly repairs.
 
Bob Turner said:
Nope. But I am not going to admit to having patched over Butyrate without going to silver or fabric. Not admitting to doing it in the slipstream on my Decathlon, even though if I did it, it was 17 years ago. But if I had done it, I probably could have reported that you can hardly see it, and that it never pulled off. Certainly couldn't admit that multiple repairs for balun antenna and changed N-numbers were involved, let alone heavy duty belly repairs.
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You weren't an over-the-road trucker in a former life were you? 🤐🤫🤭
Chris
 
That contact cement is part of the approved covering process for the Citabria. Makes it really hard to pull the old cover off.

Could it be Super Glue? Or Gorilla glue? Or maybe T-88? But the real question: is it holding?
 
Bob Turner said:
That contact cement is part of the approved covering process for the Citabria. Makes it really hard to pull the old cover off.

Could it be Super Glue? Or Gorilla glue? Or maybe T-88? But the real question: is it holding?
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Looks like it is holding fine. But I'd like to determine what it is, in case I want to topcoat it with colored dope at some point.
 
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