Some people feel that everything must have some sort of thread locker on it. So I'd recommend the weakest of all, clear nail polish, for this. In reality, the way the buttstock assembly goes together puts enough tension on the buffer tube that is not going to go anywhere. The stock fits against both the receiver and the buffer tube, effectively locking the buffer. But, some people insist on some sort of thread locker. Just don't use LocTite. Please.