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When applying pipe joint compound, a.k.a dope, does the application of tape (Teflon tape etc...) not hinder a chemical bond between the compound and the threads, or is does the compound merely serve as a filler, albeit one that withstands high pressure?

Here is a new threaded water fitting where I noticed a slow leak:

enter image description here

I have a suspicion that the problem here is too much tape (someone did this before me), and I will take it apart and redo.

But what's interesting is that it seems there is water escaping at both ends of the threaded connection. This suggests the water is traveling under the tape to the far end as well.

Could pipe compound over tape have avoided this? Does this not make the case for compound-only?

This question, What goes on threads first: tape or dope? has answers either way, and comments arguing for or against each. It all seems based on valuable experience, but there is no mentioning of the science. And youtube videos on this topic go either way, or are indifferent about it.

To clarify the specific question in light of that and other similar questions: if the water is leaking under the tape, is compound -which goes over the tape per manufacturer's recommendation- the right remedy?

But I think the answer is in the comments: the taping is poorly done, and either it should be re-done, or it should be doped. No need to do both, this is an application failure, not the wrong choice of method.

Is this question a duplicate? No.

Other questions (see here and as linked in the comments), address when either of the two are required, yielding many contradicting answers.

My specific question asks about what happened here in my fitting (drops despite tape, no dope), could it be avoided with dope, and wouldn't dope alone be better than tape since tape is the point of failure here. And whether the tape would hinder a chemical or better compound bond (no it doesn't, a compound seal is not based on a chemical bond).

There are many valuable insights in the comments and in the answers, some reflect the sentiments and facts in previous questions and some are new.

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    As a non expert, I always thought you used one or the other, depending which one was handy. With the dope you don't need to know which direction to wrap the tape, just brush it on.
    – crip659
    Commented May 19, 2021 at 18:11
  • Wahahaha! Oh no, next: the tape direction debate... Anyway, what you write is what I thought until I shopped around, and now I am none the wiser. I thought tape hinders the bond... Let's see what the answers say.
    – P2000
    Commented May 19, 2021 at 18:15
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    My plumber buddies always "double dope it" IE: Apply the sealing compound (dope) first, then a couple of wraps of teflon tape. I've done that and never had a leak.
    – George Anderson
    Commented May 19, 2021 at 18:35
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    Does this answer your question? What goes on threads first: tape or dope?
    – isherwood
    Commented May 19, 2021 at 19:59
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    I suspect you're right--too much tape. This would cause it to bunch up and push out rather than staying in place between the fittings.
    – isherwood
    Commented May 19, 2021 at 20:07

3 Answers 3

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You're supposed to use one or the other. If you're going to use both then apply the tape first. If you dope then wrap then I could foresee the tape being pushed out of the way before it gets gripped in the female threads.

Neither one is designed to produce a chemical bond, not sure where you got that idea from. These products aren't Loc-tite blue/red.

Both are designed to act as physical obstructions so that water cannot get through the threads. I don't think that using both hinders nor helps to stop leaks over using just one; proper material coverage and tightening are your best friends.

Some plumbers like to use both for whatever reason; old habits die hard I guess.

Maybe they've succumbed to some persistent inspector that likes to see both products applied so now it's a habit and propagated de facto. Here's an HVAC instructor that tells his class to "apply mastic to the outside of flex duct to appease the inspector". Obviously this example is HVAC but the common denominator is the same; the inspector.

I have success regardless of which method I use.

I have a particularly rusted galvanized line to which I ran PEX. The threaded brass transition leaks a tear-drop every so often but it's actually caked with enough rust/hard water that it no longer leaks. I may try switching out the pipe dope for tape whenever I have some time to spare.

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  • According to Oatey: "Once the male threads are wrapped properly you can apply a coating of pipe joint compound over the thread sealant tape before assembling the joint."
    – P2000
    Commented May 19, 2021 at 19:16
  • @P2000 Well that's interesting. I have an old-ish jar of their dope so I'm gonna read it when I get home. Regardless, it says "Tape and joint compound can be used in conjunction with one another." There is no explicit recommendation to do both. However, if you choose to do both then they do suggest that the tape does go on first.
    – MonkeyZeus
    Commented May 19, 2021 at 19:22
  • Just wondering if this is the same as shampoo, they double their sales by adding the word 'repeat'. Does Oatey make tape also?
    – crip659
    Commented May 19, 2021 at 19:28
  • @crip659 I would bet they've received a fair share of phone calls on the situation that they decided to do their own testing and add it to the label. I wouldn't be surprised if their automated greeting included "Read the label for the tape+dope debate and hang up or press 4 to continue"
    – MonkeyZeus
    Commented May 19, 2021 at 19:31
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    Oatey probably says that because some folks insist on using both. It makes no sense to me. Why wouldn't you want the thread compound to seal to the fitting, and not to the tape? It's like putting a flat washer under a lock washer.
    – isherwood
    Commented May 19, 2021 at 20:02
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What really does work in conjunction with pipe dope is plumber's hemp. This is used widely in Europe, New Zealand, and Australia, perhaps because they sometimes use parallel threads which cannot be sealed completely with dope alone. Plumber's hemp will also work with the tapered threads used in the US. I have never used hemp, but I intend to try it in the future, and I do not intend to scratch the male threads to hold the hemp in place.

There are numerous videos showing how to use plumber's hemp and they advocate different methods of use. Here is just one:

https://youtu.be/77QxXewSVbg

enter image description here

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    First I thought this was a joke... dope & hemp. There is something like this in North America, called pipe wick. Comes on a roll like dental floss or yarn, unlike a big blond hemp wig as shown in the video. Isn't hemp more for high temperature applications like heat & steam?
    – P2000
    Commented May 20, 2021 at 4:09
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    Modern pipe dope contains granules which replaces the need for hemp/twine/wick. I am pretty sure you have to buy a specific dope to be used with this string method which does not contain granules.
    – MonkeyZeus
    Commented May 20, 2021 at 11:43
  • Yes, the paste contains no granules. It's pretty thick. Make sure you get the one specified for drinking water fittings, that one is non-toxic. I've never had a leak with that, it's very reliable, but a bit messy. There's a kind of tape that does the same thing, it's fiber impregnated with dope, called "atmo seal". It's excellent, faster, and less messy. Also you will need to scratch the male threads.
    – bobflux
    Commented May 20, 2021 at 19:04
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Honestly the real issue is too many fittings/leak points. It looks like they either could not find the proper pex to threaded fitting or used what they had on hand. Find the proper fitting and reduce the leak points.

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