U tube? I have not heard that proper English since I was in England.
I bet the leak is on a portion where the waste is traveling up, no?
Well, there are two things you could try. The first is to buy a new seal which is the plastic or rubber washer you should have found when you took the trap off. These are not supposed to be reusable.
The second, if it is metal pipe that you are using, is to tighten down a little more.
You should not need to use teflon tape or putty in sealing a trap.