This article will explain the difference between thread tape and thread paste.
Thread Tape
Often referred to as a plumber's best friend, Teflon tape or thread sealant is a simple, yet effective material found in kits worldwide. Also known generically as PTFE tape, it's generally used as a sealant and lubricating material for fittings.
An important thing to remember is that PTFE tape should not be used when connecting PVC fittings or valves with a female (FPT) thread. If the tape is used on female connections, a wedging action can occur which will cause major stress on the joint during assembly and could crack or pop the connection. PTFE tape does not guarantee a leak-free connection.
Strictly speaking, Teflon tape is not a “thread sealant”. The tape may have the effect of clogging the thread path, but it does not actually adhere to surfaces as a true sealant should. During installation, the tape must be carefully wrapped in the direction of the threads, or it unravels and tears.
Thread Paste
The better substitute for plumber's tape (thread seal tape) is “pipe dope,” also known as pipe thread sealant or pipe joint compound. Thread paste is specifically designed to seal the threads of a pipe and is known to be better for maintaining a permanent steal than Teflon plumber's tape.
When working with fittings with straight threads, thread paste should always be used. Thread tape is too thick and can prevent a good seal in straight thread joints. Thread paste will spread and be pushed out of the way and will not prevent the threads from properly engaging.
Pipe joint compound is easy to find, fairly simple to use, and inexpensive. It works with all types of pipe and fitting materials, and it produces a strong seal. Pipe dope is generally a stronger seal than Teflon tape, which is why plumbers and other professionals use it rather than tape for seals that are permanent.
Application
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.
If you use tape, you can also apply the paste to the joint as well. Go ahead and wrap Teflon tape around the male threads just like always. Then add pipe thread sealant as shown. The pipe thread sealant will fill any gaps caused by tape failures. This will help prevent a leak.
Don't use Teflon tape for metal pipes and fittings. Metal-to-metal fitting joints are more difficult to tighten than plastic and need Teflon paste. The surfaces tend to gall without the aid of such lubricants as Teflon or pipe dope.
For Stainless Steel, LOCTITE® red thread locker is the highest strength of thread locker adhesive. This product cures fully in 24 hours and is available in both a liquid and as a semisolid anaerobic. The red high-strength thread locker products are so powerful they may require heat to be disassembled.