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Situation: rough in valve with 1/2 in NPT. I sealed the threads with Teflon tape and some pipe dope (probably not enough). However after turning on the water, there are some threads with leaks. Can I unscrew the threads, remove Teflon+dope*, and reseal it again?

* Pipe dope can be removed using some things, such as solvents, vinegar or heat from what I could find. Are these sufficient to make the thread clean again so it can be properly resealed?

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    I believe you use pipe dope or teflon tape. Not both together. Removing both and then using just one should work. Add a picture of the fitting/threads. Some thread types do not need/use thread sealer.
    – crip659
    Feb 15 at 12:18
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    many people use both and have success, but many also use only one,
    – Jasen
    Feb 15 at 12:59
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    I also use only one. It may work in many cases to use both, but it makes no logical sense. You want to reduce the number of interfaces requiring a seal, not add to them. For iron pipe and copper I use thread compound because it's more forgiving of roughness and debris.
    – isherwood
    Feb 15 at 14:32
  • Tape is for shower heads. That's it. The only way to fail using dope is getting it all over the first and second threads, which then squeezes into the pipe.
    – Mazura
    Feb 16 at 2:26

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It's not necessary to thoroughly clean a fitting of prior sealant before re-use. Remove any chunks of tape or dried pipe dope, but don't worry about wiping away the pipe dope residue.

Teflon tape doesn't really seal the pipe; it's more of a dry lubricant. The seal is accomplished by tightening the joint until the threads jam tightly into each other. The Teflon tape gets cut, crushed, and forced out of the way in the process.

There are many grades of Teflon tape. Some are very lightweight - they're nearly transparent and easily float on the gentlest current of air or static electric charge. This grade is, if I may be so blunt, garbage. When a roll of tape is included as an accessory with a package of plumbing or air fittings it is usually of this caliber. It's difficult to keep this grade in place, and to get enough of it built up, to be useful.

The best Teflon tape I've used is the Blue Monster brand. There surely must be others of similar quality. What sets these apart is that the Teflon film is much thicker (heavier). Even still the instructions recommend to apply three full wraps.

When I have a joint that leaks for the re-do I'll often switch to a pipe dope product that includes Teflon (PTFE). Oatey Great White is one example. As a heavy paste pipe dope really does help plug small imperfections. Any dope can act as a lubricant too, but the dope with Teflon is noticeably better in that regard.

Application of Teflon tape first and then dope is not wrong but normally is excessive. I will say that I've had one joint on a 1" natural gas system where the threads had been cut extremely poorly but tape plus dope and a good hard wrenching got the job done.

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    Second this. I have yet to see a pipe thread leak that couldn't be solved with a wrench. (Now, compression fittings are another matter....) Feb 16 at 2:19
  • solved with wrench +1. Teflon is lubrication for tapered pipe threads. It is not a sealant.
    – Mazura
    Feb 16 at 2:22
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Yes, these sealing products can be removed by mechanical means like picking at it or using a wire brush or pot scourer. Chemicals (like vinegar) are not needed.

Then fresh product can be applied and the joint re-attempted,

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I have had great results using this pipe thread sealer alone.

Remove the teflon tape and wipe away the old sealer. The threads do not have to be virgin clean. Apply the new sealer and tighten the fittings.

This method has a better than 90% success rate for me. I know some use the teflon tape as well, but I do not recommend it.

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Applying Teflon tape and pipe dope together doesn't make any sense. It's like applying lubricant on a joint before caulking it. Teflon tape is the lubricant in this framework. It allows better engagement for tapered threads like NPT. NPT's taper make the threads self sealing, so the lubricant alone is enough. Pipe dope is an actual sealant. You can use it on straight threads in addition to tapered threads like NPT.

Disassemble the joint and remove the tape and any pipe dope that you can. If you use any chemicals and you're working with metal fittings, be sure to clean everything well afterwards. Especially copper will corrode with residual vinegar, for instance. Residue from some chemical is more likely to disrupt the next attempt than residual pipe dope, so you should probably limit yourself to mechanical methods for removing the old stuff.

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