Sealing tanks is not rocket science. But it does require absolute cleanliness and people who are totally dedicated to doing a good job.
Every trace of the old sealant needs to be removed, with the surface repaired from any scraping/removal process, and the priming must be 100%. A years warrantee may be fine, but from my days I recall it lasting many years.
The RAAF has a program of continually monitoring the tank sealant, as we learned the hard way with our C130A models, that cutting the corners comes back to bite you.
Of course if you have people supervising who have little C130 experience, then the workers who also may lack experience will not get checked when things are going wrong.
It looks to me from outside, that the aircraft are working hard, maybe too hard, and so when the maintenance program goes by the board, it always comes back to haunt you. Then you end up with more aircraft in the shop, than are available for work.
Sounds to me that you may have a real problem hiding in the wings.
Regards
Col Tigwell