Evidence grade: B2 — Practitioner rule of thumb
Answer first: A threaded joint leaks because of the spiral clearance between the male and female threads — sealant's job is to plug that gap, not to glue the joint together. For tapered water-pipe threads, PTFE tape or pipe dope (thread-sealant paste) both work when applied to clean, undamaged threads; epoxy on the outside is an emergency hold only. If the threads are damaged or the leak survives a proper re-make, replace the fitting.
Effort: Easy–Moderate · 20–40 minutes · two wrenches, wire brush, PTFE tape or pipe dope, rags, bucket
What the sealant is actually doing
Every threaded joint has an inherent leak path: the clearance between the crest of the male thread and the root of the female thread, a spiral gap created by manufacturing tolerances. Pipe threads have no other sealing method (no flare, no O-ring), so they rely on a sealant to plug that spiral path. The taper of the threads provides the mechanical wedge; the sealant fills the gaps the metal can't [1].
Two practitioner truths follow from this. First, sealant is not a substitute for good threads — damaged, galled, or out-of-round threads will weep no matter what you wrap them in, and overtightening causes more thread leaks than undertightening (it strips threads the same way over-torquing strips a bolt) [1]. Second, tape and dope are doing the same job by different means: tape is clean and fast on small metal fittings; paste fills larger imperfections and is preferred by many pros for bigger diameters, at the cost of mess [1].
The decision
PTFE tape (Teflon tape) — for tapered metal threads in water plumbing. Clean the threads, skip the first thread at the pipe end, and wrap 2–3 layers in the direction of the threads (clockwise as you face the pipe end) so tightening doesn't unravel it. Don't let tape overhang into the pipe interior — on systems with fine orifices or valves, shredded tape is a contaminant, which is why many industrial practices forbid tape on hydraulic systems entirely [1].
Pipe dope (thread-sealant paste) — for larger diameters, rougher threads, or when tape has already failed twice. Brush or finger-apply to the male threads, keeping it out of the pipe interior. It's messier than tape (and permanent on clothing), but many experienced fitters reach for it first and never go back [1]. Use only dope rated for the fluid and material — some formulations aren't compatible with certain plastics.
Epoxy on the outside of the joint — emergency only. It can't reach the spiral leak path inside the threads; at best it dams the weep until you can disassemble and re-make the joint properly. See Repair Lab O9 for the putty procedure and O10 for the comparison.
The combination question. Some pros use tape under dope; others consider it redundant and messy. Either is defensible — what's not defensible is stacking products to avoid disassembling a joint with damaged threads.
One material-specific caution, reported from practitioner forums citing fitting-manufacturer guidance: on PVC threaded fittings, manufacturers recommend thread-sealant paste rather than tape — tape adds solid material that can wedge a molded taper apart — with no more than about 3 wraps if tape is all you have in an emergency, and hand-tight plus roughly a half turn, never wrench-tight (practitioner-forum report citing manufacturer guidance, opened 2026-09-30 — still verify against your fitting manufacturer's instructions) [2].
Stop rules
- You smell gas — leave the property and call the gas company or emergency services from a safe location. Don't operate or relight gas appliances near a leak [3].
- The joint can't be disassembled (no union, buried in a wall, no working shutoff upstream) — an outside epoxy hold is the most you can do; plan the proper repair with a plumber.
- Threads are visibly damaged, cracked, or the female fitting is split — sealant won't save it; replace the fitting. Overtightening a cracked female fitting (especially PVC) only finishes the job.
- The leak survives a proper disassemble-clean-re-tape — the threads or the fitting are the problem, not the sealant. Replace.
- On gas lines, use only sealants rated for gas service — and a gas leak you can smell is a call-the-gas-company event, not a DIY re-tape.
PPE and ventilation
No special PPE for tape. Wear gloves with pipe dope (it's tenacious), and eye protection when wire-brushing threads. Some dopes are solvent-based — ventilate the work area and follow the product label. No respirator needed for ordinary water-plumbing thread sealants used as directed.
Honest limits
- This covers tapered pipe threads (NPT-style) on water lines. Straight-thread fittings seal with washers or O-rings, not with thread sealant — taping those is a category error [1].
- The PVC guidance above comes from practitioner forums citing manufacturer guidance — verify against your fitting manufacturer's instructions before trusting it.
- Sealant choice is genuinely a matter of fitter preference within the rules above; anyone selling you "the one true method" is selling something.
- A threaded joint that has been weeping for months may have corroded the threads themselves — inspect before re-making.
Sources
- TractorByNet forums — "Pipe Tape vs Pipe Dope in High Pressure Hydraulics" (practitioner thread: sealant plugs the crest/root clearance spiral path; tape contamination risk on fine-orifice systems; overtightening vs. undertightening; tape-vs-paste preferences; skip-first-thread wrapping technique). URL: https://www.tractorbynet.com/forums/threads/pipe-tape-vs-pipe-dope-in-high-pressure-hydraulics.371442/
- TroubleFreePool forums — "Sealing threaded connections - what's the secret?" (opened and read 2026-09-30; practitioner thread citing PVC fitting-manufacturer guidance: paste preferred over tape, ≤3 wraps in emergencies, hand-tight plus ~1/2 turn). URL: https://www.troublefreepool.com/threads/sealing-threaded-connections-whats-the-secret.68771/
- JMJ Plumbing — "Emergency Plumber for Burst Pipe: A Homeowner's Guide" (gas-smell evacuation guidance). URL: https://jmjplumbing.com/emergency-plumber-for-burst-pipe/
Repair Lab · Release 1 · Evidence last checked September 30, 2026 · A project of AOJEL