Pipe Thread Sealants: Teflon Tape vs Pipe Dope | Apex Flow

Tapered pipe threads (NPT) seal on the threads themselves, and the small spiral gap between the thread flanks must be filled to stop leaks. That is the job of a thread sealant — either PTFE thread-seal tape ("Teflon tape") or a paste sealant ("pipe dope"). Choosing the right one, and applying it correctly, is the difference between a dry joint and a slow weep. This guide compares the two, breaks down the PTFE tape color codes, and shows exactly when each is required — and when no sealant belongs at all.

Apex Flow Solutions supplies brass and pipe fittings along with the thread sealants that go with them. Use this guide to match sealant to service.

Sealing a gas or oxygen line?

Sealant choice is safety-critical on fuel gas and oxygen service — the wrong tape can fail or react. Contact our team for the correct rated sealant before you assemble.

When Sealant Is (and Isn't) Needed

Thread sealant belongs only on tapered threads that seal on the threads — NPT and NPTF (and the tapered British BSPT/R). It does not belong on connections that seal another way: compression ferrules, 45° and 37° flares, push-to-connect O-rings, and BSPP/SAE straight threads that seal on a washer or O-ring. Adding tape or dope to those sealing surfaces can actually prevent proper contact and cause the leak you were trying to avoid. The single exception within those fittings is when one end carries an NPT thread — seal that NPT end only.

PTFE thread seal tape and a brush-cap pipe dope sealant next to a threaded fitting

Two ways to seal an NPT thread: PTFE tape wrapped into the threads, or paste pipe dope brushed on. Both fill the spiral gap between thread flanks.

Teflon Tape vs Pipe Dope Comparison

Attribute	PTFE Tape (Teflon)	Pipe Dope (Paste)
Form	Wrapped film	Brushed-on compound
Lubricates threads	Some	Yes — eases assembly, reduces galling
Best on large threads	Harder to keep aligned	Yes — fills irregularities well
Clean / no drips	Yes — dry, no mess	No — can drip, needs cure time on some types
Immediate pressure	Yes	Non-hardening: yes; hardening: after cure
Plastic (PVC/CPVC) threads	OK, but use dope rated for plastic	Use only plastic-safe (non-petroleum) paste

PTFE Tape Color Code Reference

PTFE thread-seal tape is color-coded by density and intended service. Using the right grade matters — standard white tape is too thin for large gas threads, and only oxygen-cleaned tape is safe on oxygen lines.

Tape Color	Service	Notes
White	General water / air	Standard density; most common
Yellow	Fuel gas (natural gas, propane)	Higher density; gas-rated
Pink	Water (heavy-duty)	Thicker; for larger water threads
Green	Oxygen service	Oil-free, oxygen-cleaned — never substitute
Gray	Stainless / anti-seize	Contains nickel to prevent galling

Which to Use by Application

Application	Recommended Sealant
Household / small water threads	White PTFE tape
Large-diameter water / process	Pipe dope, or pink tape + dope
Natural gas / propane	Yellow gas-rated tape or gas-rated dope
Oxygen lines	Green oxygen-rated tape only
Stainless threads (anti-galling)	Gray nickel tape or anti-seize dope
PVC / CPVC threaded	Plastic-safe (non-petroleum) thread sealant
Compression / flare / push-to-connect	None — these do not seal on threads

How to Apply Each Correctly

PTFE tape: Start one or two threads back from the end so tape doesn't shed into the line. Wrap in the direction the fitting will turn — clockwise when viewed from the open end of the male thread — so tightening pulls the tape tighter rather than unwinding it. Use 3–5 wraps for water/air, more for gas and large threads, keeping the tape pulled snug into the thread roots. Then thread the joint and tighten.

Pipe dope: Brush a continuous film over the male threads, filling the roots, leaving the first thread bare. Thread by hand, then wrench-tighten. Non-hardening pastes can be pressurized immediately; hardening/curing types require the manufacturer's cure time before pressure. For both methods, the goal is the same NPT engagement — roughly hand-tight plus 2 to 3 turns with a wrench (see the related NPT tightening guide for size-specific turns).

Correct direction to wrap PTFE tape on a male NPT thread

Wrap PTFE tape in the same direction the fitting tightens so the threads pull it snug. Wrong-direction wraps unravel as you assemble.

Standards & References

NPT tapered threads are defined by ASME B1.20.1; the dryseal NPTF form by ASME B1.20.3. PTFE thread-seal tape for general use is specified under ASTM D3308 (PTFE tape) and the U.S. federal spec A-A-58092. Fuel gas sealants must be listed for gas service; oxygen-service sealants must meet oxygen-compatibility cleaning requirements (e.g., per CGA guidance). For potable water, the sealant should be NSF/ANSI 61 certified. Always follow the gas code and the Authority Having Jurisdiction for fuel and oxygen connections.

Frequently Asked Questions

Is Teflon tape or pipe dope better?

Neither is universally better. PTFE tape is clean and convenient for small water/air threads; pipe dope lubricates and fills irregularities better on large threads and stainless. Many pros use tape plus dope together on critical large-diameter joints.

Which way do you wrap Teflon tape?

Wrap clockwise as viewed from the open end of the male thread — the same direction the fitting turns when tightening. This makes the threads pull the tape snug instead of unwinding it.

Do compression and flare fittings need Teflon tape?

No. Compression fittings seal on the ferrule and flare fittings seal on the cone — both are mechanical seals. Tape or dope on those surfaces can cause leaks. Only seal an NPT thread if the fitting has one.

What color Teflon tape is used for gas?

Yellow PTFE tape is the gas-rated grade for natural gas and propane lines. It is denser than standard white tape. Always confirm it is listed for fuel gas service.

Can I use regular Teflon tape on an oxygen line?

No. Oxygen service requires green oxygen-cleaned, oil-free PTFE tape. Standard tape may carry oils or contaminants that pose a fire risk in oxygen-enriched environments.