As our aircraft age, so does the sealant in our fuel tanks.
The original life expectancy of the sealant is somewhere between 10 and 20 years. We are seeing planes with sealant double that age.
As the sealant has aged, it has become more rigid or brittle.
The normal vibrations and resonance of the airframe causes the sealant to break apart at the molecular level.
At this point, you start seeing blue fuel stains appear around the wing and smell fuel fumes in the cockpit.
Once the signs of leaks have occurred, this is the tell that the sealer has deteriorated and is now on the downhill run.
Patching the tanks at this point will only buy some time to schedule a reseal.