Polytetrafluoroethylene (PTFE) Tape is a type of tape popular amongst plumbers. It's mainly used to wrap around pipe threads and helps to prevent leaks from joints, for example on radiator valves or taps.
PTFE Tape also goes by the name of Plumber's Tape, Teflon Tape and Thread Seal Tape.
Applying the tape
The correct way to apply the tape is to follow the direction of the thread, as if you were tightening up the nut. 99.9% of the time, nuts tighten up clockwise, so this is the way you should also apply the PTFE tape.
For the sake of proving the point, you could wind the tape anti-clockwise onto the thread. If you are right-handed, you will probably find it quite awkward to do, but more importantly, when you try and screw the nut on, you’ll find that the PTFE will start to come off.
This obviously renders the PTFE tape completely useless and highlights exactly why you should wrap it around the thread in a clockwise motion.
When applying the tape clockwise, it’s recommended that you wrap it around the thread three or four times. This is thick enough to prevent leaks, but not too thick that you can’t get the nut on. When putting the nut on over clockwise-wound tape, you’ll see if fits over easily and the tape does not start to come off, in contrast to when it was wound anti-clockwise.