Which JB formula are you using?
I used their their plastic specific, later realized that original formula is a general purpose and is stronger. So that may have been one of the reasons why mine did not hold longer. As far as amount/coats, understand that there is a specific area where most of the tension happens and only so much JB can be packed in there to create strongest bond. It is no different to people assuming that more grease creates more lubrication, or more conductive compound creates more heat transfer, so they start smudging excessive amounts without realizing that only 1% will actually stay on the surface and do its job while rest will be squeezed out and wasted.
The reason why i did not redo the job after it has failed is because when that specific blend door moves, there is binding happening which creates significant tension in my opinion. If you take the unit out and play with it, you will feel it. Actuators are plenty strong to overcome that binding tensions, but the clips are not. That is why i did not expect JB Weld to last long.
If JB approach fails, the screw method is easy to implement. After taking the center dash out, blend door will be visible and accessible. Play with it until you find desired split of air flow between center and top vents. tap screw into the blend door and run it against the adjacent wall, this will fixate it in place and you will always have desired air distribution.
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