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I've seen them use teflon tape on the nylon fittings in factory aircraft. It's about the only place I would use teflon tape on an aircraft.
I never have done a leak test.
Where you can, like usually on straight joints, I use electrical heat shrink to seal. works good.
For clamps I just use 1/4" tie wraps. There's not much clamp force there but it's always worked for me especially if the fitting has a swell in it instead of the barb type.