Sealing Pipe Threads - Anaerobic, PTFE or Hemp?
Whether for water, gas or compressed air, tapered and parallel pipe threads must be sealed reliably and durably. This guide compares anaerobic liquid sealant, PTFE tape and hemp by pressure, medium and later disassembly, and shows which sealant fits which job.
View sealantsWhich pipe threads exist and where do they seal?
Pipe fittings distinguish sealing from non-sealing threads. A tapered male thread to ISO 7 (designation R) seals in the thread itself, while a parallel female thread (Rp) needs a sealant in the flank. Parallel threads to ISO 228 (G) instead seal via a flat gasket on the shoulder and not in the thread.
When an R thread is screwed into an Rp thread, sealing comes from the clamping flanks plus a sealant that fills the remaining spiral gap. That gap is exactly what hemp, PTFE tape or an anaerobic liquid sealant closes.
- R (ISO 7): tapered male thread, sealing in the thread.
- Rp (ISO 7): parallel female thread for pairing with R.
- G (ISO 228): parallel, seals via flat or O-ring gasket, not in the thread.
- NPT (ANSI): tapered inch thread common in North America, also sealing in the thread.
Anaerobic, PTFE or hemp - what handles what?
The three methods differ mainly in pressure rating, media resistance and how easily the joint can be undone later. Anaerobic liquid sealants cure in the thread in the absence of air and reach the highest pressure and vibration resistance.
PTFE tape (Teflon tape) is wound tightly in the thread direction and is clean, odourless and quick to apply. Hemp with a jointing paste swells slightly on contact with water and remains a proven choice for larger threads in heating and plumbing.
How do I choose the right sealant?
The decisive factors are medium, operating pressure, temperature and whether the joint has to be reopened. Anaerobic sealants come in several strength grades: low strength for small threads and easy disassembly, medium to high strength for durable, pressure-tight joints.
- Compressed air and hydraulics: anaerobic liquid sealant, medium strength, vibration-proof.
- Drinking water: use only products approved to DIN EN 751 (or local water approval).
- Gas: approved anaerobic sealant (EN 751‑2) or FRp PTFE tape (EN 751‑3).
- Plastic threads: PTFE tape, since many anaerobic products do not cure on plastics.
- Large metal threads from 1 inch: hemp with jointing paste stays practical.
Frequently asked questions
Which direction do I wind PTFE tape?
Always in the tightening direction of the thread so the tape does not unwind when screwing together. Three to five tight, slightly overlapping layers are enough on smaller threads.
When is anaerobic liquid sealant better than PTFE tape?
At high pressure, strong vibration and with media such as oil or fuel. It fills the gap completely and cures into a pressure-tight, self-locking joint that, depending on the grade, still remains removable.
Can I use hemp for gas?
Only with an approved jointing paste and within the scope of the applicable installation rules. For many gas applications, approved anaerobic sealants or FRp PTFE tapes to DIN EN 751 are the safe choice.
Can an anaerobically sealed joint be reopened?
Yes, medium-strength products release with normal tools. High-strength sealants may need heating. For frequent disassembly choose low-strength grades or PTFE tape.
The right sealant for your pipe thread?
Anaerobic liquid sealants, PTFE tapes and hemp sets - approved to DIN EN 751 for water, gas and compressed air.
Standard-tested
Sealants approved to DIN EN 751 for gas and water.
Pressure-tight
Anaerobic sealants for operating pressures up to about 350 bar.
Serviceable
Strength grades for removable or permanent joints.
Expert advice
Our specialists help you select the right product.