We have 2 identical faucet. We change the aerator of 1 without trouble. The second one is not staying in place. We tried to put some silicone around the threaded faucet, but it is not working. We are thinking that when we unscrew the aerator, the internal threaded must have some deposit and today it is too loose. What can we do ?
Take a rag soaked in lemon juice, vinegar or CLR and shove it up into the faucet against the internal threads for a few hours. Keep re-soaking the rag. Then brush the threads with a toothbrush and reinstall the aerator. Remember to use the washer that came with the aerator. Oh, don't use the toothbrush on your teeth...
You may have cross-threaded the aerator when you put it on. That would allow it to screw on 1 turn, maybe 2, but then it would jam as though it was tight, but it wouldn't be tight.
Once cross threaded, it just won't go on right anymore. You'd have to "chase" the threads on the faucet to recut and clean them up, and do the same for the aerator. Replacing the problem aerator again would probably be easier than trying to chase the threads on it.
You could try swapping the aerators from one faucet to the other, but if you have cross threaded, you risk damaging both aerators.