Teflon tape is used to seal the threads on fittings, as it gets into the gaps and is easily compressed - instead of the material your fitting is made of having to do this. Some brass fittings will seal well when tightened fully, however stainless steel fittings generally don't because the stainless steel is very hard and will not distort enough to seal the threads fully.

It is still necessary to screw the fitting in tightly to achieve a seal after using tape, however care should be taken not to over-tighten the fitting. Teflon tape can also help prevent a phenonemenon known as binding, where even matching threads will 'bind up' when tightened, and refuse to loosen off. This can quite often result in damage to the threads, the threads being removed from one fitting - or just simply not ever being able to remove the fitting from where it is attached. If this happens, sometimes the only options are to have the fitting cut out of the vessel and replaced - or leaving it where it is and hoping it never leaks....
Clearly this is something worth avoiding at all costs, particularly when the thread is in a vessel worth a lot of money.
The following sequence of photos shows a stainless hosetail being taped in preparation for being screwed into a socket fitting. Take particular note of the direction the tape is being rolled in over the thread, and how tension is being placed on the tape in order to stretch it over the thread, while being sure not to cover the opening of the fitting at all - as this would result in the flow path through the fitting being interfered with.

1.The clean fitting. Hold in your left hand, with the thread facing towards your right hand.

2. With the roll of thread tape in the position shown, hold the end of the tape flat against the thread with your left thumb while stretching the tape with the roll over the thread.

3. Continue around the thread, right around to where the tape overlaps with some tension on it. If you have done this correctly the tape will stick to itself enough once overlapped that you can let it go to finish taping the fitting.Be very careful not to pass tape over the end of the fitting, blocking the flow path.

Roll / stretch the tape around the fitting at least 6 times, then hold the tape with your left thumb again and snap the tape off carefully.

The fitting taped ready to be screwed into it's female counterpart.

Insert into female fitting, make sure thread is located properly and tighten until sealed. Be careful not to wind the fitting back at all, as the tape is a 'one shot deal' and if you undo it it may not seal properly and you will have to re-tape it.

All of your stainless fittings and some brass fittings will require this taping in order to seal. The exception to the rule is the compression nut with ferules that is on the hop screens and pickup tubes we sell for brew kettles and other vessels - don't use thread tape in these as they are designed to seal without the tape, and putting tape in them can cause the nut to spread and eventually split.

Please note that the white PTFE or teflon tape/plumbers tape/thread tape as it is known is designed only to seal threads for liquids, and is not appropriate for use with any gas fittings. Teflon tape for gas fittings is generally a distinctive colour - usually yellow.

You will need to re-tape your fittings when they are disassembled from your equipment, or if they become lose. Thread tape is not re-useable, but it is usually very inexpensive at around 60 cents a roll, so we recommend keeping a roll or two in your brew equipment box :)

Happy brewing!, Wayne.