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Leak Testing Fuel Tanks

Supports the case-in-point. A sample size of one isn’t enough to make such wide claims.While he doesn’t feel it, PaulVS probably got a little lucky. The reversion was most assuredly going to happen no matter what. Better to find out before the tanks leaked or decided to puke a bunch of goo into his operating fuel system.

I’ve spoken with the (original formulation) Desoto engineers a few times over the years. They have original sealant still doing its job since the 1950s. This was before they buttered the faying surfaces and only filleted the seams. They claim dry layup will not affect its life; though, there is incorrect tribal knowledge stating tanks must remain full. This helps from a “heat sink“ effect for drawing sun heat from the sealant, not keeping them wet = longer life. My 1969 Mooney had original sealant when I sold it a few years ago (Desoto brand) and it sat on the ramp in the FL sun for a long time before I bought it. To be clear, I’ve never spoken to the Flamemaster engineers.

Could be a mixing/application error but there have been bad production batches in the past. Either way, I still believe some luck was had when it was discovered early.

@fixnflyguy. Does your business make test/validation samples or coupons when they apply it? Wouldn’t be surprised.
We make Test Coupons for every single sealant application, be it fuel tank or fuselage assembly or repair . Sealant is used virtually everywhere on a big jet. The test coupon is the final inspection for Quality Control (Inspector) stamp and includes product batch numbers, dates, and material certs. Some sealants we use have 100 plus hours of working time, and must achieve a percentage of cure before fillet seal and return to service. I have worked with nearly every PPG, Desoto, 3M, Courtalds Aerospace and the gamut of brand names of sealants used in both Heavy and GA aircraft, with exception of Flamemaster products. I have spoken to engineers, and representatives of various manufacture on the very issue of poor and failing cures, none of whom endorse reversions attributed to exposures of normal in service wear and environment. I have seen and repaired "goo" many times, typically attributed to mixing or batch failure, but not circumstance of environment. As mentioned by others, I have seen plenty of 50 year old sealant that is in aircraft parked in the dry desrt for years and returned to service. Nearly every aircraft I have at my day job sits with tanks empty and fully vented dry tanks..we go inside them without breathing equipment. While I do NOT proclaim myself as a Know-it-all about sealants, I have been involved with them almost daily for 45 years. My RV-4 tanks were built by me, with 1422A&B sealant in 1995 and leak checked with 100LL, then emptied, never had fuel in them again until 2010 for my first engine start. I am not trying to discredit PaulvS in any way, I'm just expressing my experience with various sealants and their applications. I also understand Flamemaster has been around for a long time and has been successfully used in production, however, I have never personally used it. I have attached a publicly available Pro-Seal reference guide for those who need something to read, and what practical uses and types there are, but in no way trying to promote their sale..its just what I have been using for a loooong time.
 

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We make Test Coupons for every single sealant application…
Ive been in military aerospace for a few decades and like you, I’ve seen my share of sealant laid down. We will only coupon test sealant that goes into an area that needs to be closed prior to cure (rare). That said, QA needs to buy the final cure (tack free, time and durometer) before the assembly can be sold, and of course, the final check is the soak test. I have also seen plenty of “goo” in my day, but it’s always been a mix or batch failure - not seen an environmental failure yet.

Disclaimer: “tank rats“ are artists with a horrible job - I don’t claim to be one. But it seems to me that if you are going to have a reaction to fuel at some point, best to know that while the tank is still on the bench. The “goo story” above is even MORE of a compelling reason to soak check the tanks in my mind.
 
.... I have seen plenty of 50 year old sealant that is in aircraft parked in the dry desrt for years and returned to service. ...

Sorry for the drift but it's a good example of how amazing the related products are. Hats off to the ChemEs that developed it. An ~ 60 year service life to date in some cases, sealing petroleum products that are often "unofficially" used as solvents themselves and B-52 wings flex A LOT. Impressive to my pea brain.
 
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