Choosing the wrong pipe thread sealant is one of the most common causes of joint leaks, code violations, and even equipment damage in plumbing, HVAC, and industrial piping. PTFE tape is not a universal solution. Pipe dope is not always appropriate. Some combinations of sealant and pipe material are incompatible and will cause joint failure over time.
This guide covers the major sealant types, explains their strengths and limitations, provides material and fluid compatibility guidance, and helps you select the right sealant for the application. It is organized as a decision framework: start with what you are sealing, and the guide narrows down your options.
How Pipe Threads Actually Seal
Not all threads seal the same way. Understanding the sealing mechanism determines what sealant (if any) you need.
NPT (National Pipe Taper) threads are tapered at 3/4 inch per foot (1 degree 47 minutes per side). As the male thread is screwed into the female fitting, the taper creates interference that compresses the threads together. The sealant fills the microscopic gaps in the thread roots and crests where metal-to-metal contact is imperfect. The seal is primarily mechanical (thread interference) with the sealant providing a backup for the spiral leak path.
NPS (National Pipe Straight) and BSP parallel threads are not tapered. They do not create interference, so the threads alone cannot seal. These connections require an O-ring, gasket, or compression mechanism for the seal. The threads only hold the joint together; the seal is on a separate surface. Adding PTFE tape or dope to straight threads does not create a reliable seal.
BSP tapered threads (BSPT) work like NPT but with a different thread form (Whitworth profile with 55-degree angle versus 60 degrees for NPT). BSP and NPT threads are not interchangeable despite looking similar. Attempting to connect BSP to NPT results in poor thread engagement and leaks.
Compression fittings, flare fittings, and mechanical couplings do not use thread sealant. The seal is on the ferrule, flare, or gasket. Adding sealant to these connections can prevent the mechanical seal from forming properly.
Pipe Thread Sealant Guide
Select the correct pipe thread sealant based on thread type, pipe material, fluid service, pressure, and temperature. Covers Teflon tape, pipe dope, anaerobic compounds, and PTFE paste.
PTFE Tape: Types, Application, and Limitations
PTFE (polytetrafluoroethylene) tape is the most widely used thread sealant. It fills thread gaps with a thin layer of PTFE that acts as both sealant and lubricant. The lubricant action allows the threads to tighten further, improving the mechanical seal.
Standard white PTFE tape (MIL-T-27730A) is 0.003 inches thick and suitable for water, air, and many chemicals up to 500 degrees F and several hundred PSI. Apply 3-5 wraps clockwise (when looking at the end of the male thread) so the tape does not unwind during assembly.
Yellow gas-rated PTFE tape is thicker (0.004 inches) and denser than standard white tape. It is specifically listed for natural gas and propane service per ANSI/UL standards. Most jurisdictions require gas-rated (yellow) tape on fuel gas piping. Standard white tape is not code-approved for gas in many areas.
Pink plumber's tape is a thicker, higher-density PTFE tape designed for larger pipe sizes and higher pressures. It is commonly used on 1-inch and larger water piping where standard tape may not provide adequate thread fill.
PTFE tape limitations: It can shred and clog small orifices, strainers, and control valves if pieces break off during assembly. For instrumentation connections, instrument-grade paste sealant is often preferred over tape to prevent contamination. On oxygen service, PTFE tape is acceptable because PTFE is non-reactive with oxygen, but petroleum-contaminated tape is not.
Pipe Dope (Paste Sealants)
Pipe dope (thread sealing compound, pipe joint compound) is a paste applied to pipe threads before assembly. It fills thread voids more thoroughly than tape, especially on large-diameter and damaged threads. Most pipe dope contains PTFE particles or fibers suspended in a carrier paste.
Advantages over tape: Better for large pipe sizes (1.5 inch and up) where tape can be difficult to apply uniformly. Handles damaged or corroded threads where tape cannot conform to irregular surfaces. Does not shred or contaminate downstream equipment. Easier to apply on vertical and overhead joints where tape can slip.
Disadvantages: Messy to apply and clean up. Some formulations stain hands and clothing. Shelf life is limited (typically 2-5 years). Some formulations are not compatible with plastic pipe (petroleum-based carriers attack PVC and CPVC).
Common types: Standard pipe dope with PTFE (for general-purpose water, air, and oil). Heavy-duty pipe dope for high-pressure gas and steam (rated to 10,000+ PSI and 500+ degrees F). NSF 61 certified pipe dope for potable water. Oxygen-compatible pipe dope for medical gas and industrial oxygen systems (no hydrocarbon carrier).
Anaerobic Thread Sealants
Anaerobic sealants cure in the absence of air when confined between metal surfaces. They are liquid when applied and solidify to a tough plastic within the thread engagement zone. The cured sealant is resistant to vibration loosening, which makes anaerobic sealants popular for hydraulic fittings, pneumatic connections, and machinery.
Advantages: Excellent resistance to vibration loosening. Fill all thread gaps completely. No shredding or contamination. Good chemical resistance. Available in different strengths (removable, medium, and permanent).
Disadvantages: Require clean, oil-free metal surfaces for proper curing. Cure time is 4-24 hours for full strength (handling strength in 15-30 minutes). Do not work on plastic threads. May require heat or special tools for disassembly on permanent-strength grades.
The most common anaerobic thread sealant is Loctite 545 or equivalent (medium strength, general purpose, rated to 300 degrees F and 10,000 PSI on steel). For high-temperature applications, products rated to 400+ degrees F are available. For hydraulic systems, use products specifically rated for the hydraulic fluid in use.
Material and Fluid Compatibility
Selecting a sealant that is incompatible with the pipe material or the fluid creates problems ranging from slow leaks to catastrophic joint failure. The critical incompatibilities to know:
PVC and CPVC: Never use petroleum-based pipe dope or sealant. The petroleum solvents cause environmental stress cracking in the plastic, weakening the fitting and eventually causing it to split. Use only PTFE tape or PTFE-based paste sealants labeled for plastic pipe.
Stainless steel: Use nickel-based anti-seize or PTFE paste on stainless threads to prevent galling. Standard steel-based anti-seize causes galvanic corrosion on stainless. PTFE tape works but does not prevent galling on tight joints.
Oxygen service: Never use any hydrocarbon-based sealant (standard pipe dope, petroleum-based anti-seize, or contaminated PTFE tape). Hydrocarbons in the presence of high-pressure oxygen can auto-ignite. Use only oxygen-compatible sealants specifically rated for O2 service.
Potable water: All sealants must be NSF 61 certified. Most PTFE tape is NSF 61 listed. Many pipe dope products are certified, but check the label. Using a non-certified sealant on a potable water system is a code violation.
Natural gas and propane: Use sealants specifically listed for fuel gas per ANSI/UL standards. Yellow PTFE tape and gas-rated pipe dope are both acceptable. Standard white tape may not meet code requirements in your jurisdiction.
Pipe Thread Sealant Guide
Select the correct pipe thread sealant based on thread type, pipe material, fluid service, pressure, and temperature. Covers Teflon tape, pipe dope, anaerobic compounds, and PTFE paste.
Sealant Decision Framework
Use this framework to narrow your selection:
Step 1: Is the thread tapered (NPT, BSPT) or straight (NPS, BSPP)? If straight, use an O-ring or gasket seal. Thread sealant alone will not work. If compression or flare fitting, no sealant needed.
Step 2: What is the pipe material? If plastic (PVC, CPVC), eliminate petroleum-based products. If stainless, add anti-galling protection. If cast iron, any standard sealant works.
Step 3: What is the fluid? Gas service requires listed products. Potable water requires NSF 61. Oxygen requires oxygen-compatible only. Chemicals require compatibility verification with the sealant manufacturer.
Step 4: What are the pressure and temperature conditions? Standard PTFE tape handles up to 500 degrees F and several hundred PSI. For higher conditions, use rated pipe dope or anaerobic sealant. For very high pressure (over 5,000 PSI), anaerobic sealants are often the best choice.
Step 5: Is vibration a concern? If the joint is subject to vibration or thermal cycling, anaerobic sealant provides the best resistance to loosening. PTFE tape and pipe dope can slowly work out of vibrating joints.
When in doubt, PTFE tape is the safest general-purpose choice for tapered threads in water and air service below 300 degrees F. When PTFE tape is not appropriate, PTFE paste (not petroleum-based dope) is the next broadest option.