For threaded plumbing connections, you typically use pipe dope or thread tape (aka teflon tape). Are there situations when one is preferred over another? Does the size of the pipe, type of pipe, part being connected, or other variable make a difference in the selection?
Note, this assumes you aren't using both at the same time.
Whether dope is required at all, is dependent on the type of threads. The type of thread is determined by the tap or die used to create the threads, and should be labeled on the pipe or fitting.
This type of thread when mated, may contain slight gaps between the major and minor diameter of the threads. Because of this, a thread seal agent may be required. However, this type of thread is tapered at a rate of 1/16 (3/4"/foot), which often allows a seal to be made without a sealing agent.
May be listed as MNPT or FNPT, for male and female threads respectively.
NPTF threads are designed so that when the threads are mated, they actually deform to create a mechanical seal. Since the seal is created by the threads themselves, a thread seal agent is not required (though may be used as a lubricant).
May be listed as MNPTF or FNPTF, for male and female threads respectively.
This type of thread is similar to NPT, except that the thread does not have a taper. A thread seal agent is required, but should be selected differently due to the lack of taper.
May be listed as MNPS or FNPS, for male and female threads respectively.
NPSF like NPTF creates a mechanical seal due to the deformation of the threads, however, unlike NPTF NPSF threads are not tapered.
May be listed as MNPSF or FNPSF, for male and female threads respectively.
Which form of dope you choose is often based on who you learned from, personal preference, what's on hand, what type of pipe you're working with, etc. The following are my personal guidelines.
In this situation I'll reach for PTFE tape, since it's not likely I'll require long term corrosion protection. I'm just looking to lubricate the joint, so I can tighten it to create a leak free joint.
Because I don't want the threads to rust or react negatively, in this situation I'll use paste dope. The paste dope will give me the lube I need to get the pipes properly joined, while at the same time providing corrosion protection. The paste dope will never harden or flake off, so I know the threads will be protected for a long time.
If I'm working with water pipe of any material larger than ~1", I'll always use paste dope. I don't really have any particular reason for this, it's just the way I was taught.
When working with fuel gas pipes of any kind, I always use a paste dope labeled for this use. This is especially true when working with "black" pipe, since I want some corrosion protection in these joints.
For pipes and fittings with tapered threads, I'll simply follow the guidelines mentioned above.
When working with fittings with straight threads, paste dope should always be used. Tape dope is too thick, and can actually prevent a good seal in straight thread joints. Paste dope will spread and be pushed out of the way, and will not prevent the threads from properly engaging.
If you're only going to use one or the other, I would use:
Rule of thumb, even though I'm sure some will disagree:
Dope on course threaded pipe, galv, black pipe etc. Also on larger diameter pipes and always special oil resist dope on fuel. oil and gas (propane,natural gas) pipes.
Tape on small 1/2 and 3/4 inch tubing and plastic fittings. Tape is almost always used on most small brass fittings for water and air. Never use dope on plastics, strictly tape.
I notice that some of the old time plumbers tend to use pipe dope on more applications than newer plumbers. Not sure if that is because they think it works better or just that it is what they have always used before tape was commonplace. Now Pex and compression tools taking over.
This is strictly an answer for if you are running air lines. I will always use tape. I've used dope in the past on black pipe and galvanized, and there is almost always a leak, even if it is tiny. An old retired pipe fitter from the pipe fitters union told me never use dope on any king of gas or air system always used tape. I have used tape ever since and it has worked perfectly every time.
Sealing depends on what you are doing: Ordinary hardware store pipe has NPS threads ( in the US). NPS has a spiral leak path that must be filled with something to hold pressure ; for pressures of municipal water and gas teflon tape and pipe dope are satisfactory. Oil /gas wells use API 8 round thread ( similar to NPS) , API pipe dope is used , it contains metal powders ( lead, zinc, copper) in grease to block the leak path at high pressures. I like the Teflon tape for any home application.
The correct answer is to use pipe dope the Teflon tape over the pipe dope. Teflon tape is not a sealant of any sort is only a thread lubricant. A true pipe thread sealer is pipe dope. As I just asked a sprinkler fitter why he was pipe doping then Teflon taping all pipe threads and that was the answer he gave me. Then I researched Teflon tape and this is the wiki page for Teflon tape.
Thread seal tape (also known as PTFE tape or plumber's tape) is a polytetrafluoroethylene (PTFE) film for use in sealing pipe threads. The tape is sold cut to specific widths and wound on a spool, making it easy to wind around pipe threads. It is also known by the genericised trade-name Teflon tape; while Teflon is in fact identical to PTFE, Chemours (the trade-mark holders) consider this usage incorrect, especially as they no longer manufacture Teflon in tape form. Thread seal tape lubricates allowing for a deeper seating of the threads, and it helps prevent the threads from seizing when being unscrewed. The tape also works as a deformable filler and thread lubricant, helping to seal the joint without hardening or making it more difficult to tighten, and instead making it easier to tighten.
Typically the tape is wrapped around a pipe's thread three times before it is screwed into place. It is commonly used commercially in applications including pressurized water systems, central heating systems, and air compression equipment. Source
