Thread Sealant Selection: Flax and Paste, PTFE Tape, Anaerobic, and Wound Cord

Six methods dominate threaded-pipe sealing, and the sensible choice depends on the joint in front of you rather than on habit. ## The options **Flax tow and jointing paste** — best for high temperature, old or damaged or oversized threads, cast iron and steel, and joints subject to vibration. Cheap, gap-filling, and uniquely adjustable after tightening. Slow, skill-dependent, seizes over years, and rots if the paste is skipped. See Sealing Pipe Threads with Flax Tow and Jointing Paste. **PTFE tape** (ФУМ-лента, thread seal tape) — clean, fast, no cure time, and the joint comes apart later without a fight. It shreds on large or rough threads, is generally kept below about 120 °C on heating work, and struggles above roughly 50 mm diameter. See PTFE Thread Seal Tape: Colour Codes, Wrap Direction, and Limits. **Anaerobic sealant** (Loctite-type thread sealant) — cures in the absence of air between metal surfaces, giving a strong, clean, chemically bonded seal. It requires new, clean, degreased metal threads: on an old dirty fitting it will not grip, which rules it out for most repair work. **Wound sealing cord** (Loctite 55 and equivalents) — a PTFE-based cord dispensed off a reel. Clean like tape, gap-filling like fibre, works well on stainless and uneven threads, immediately pressure-tight and still adjustable. The tidy modern answer where budget allows. **Pipe dope / jointing compound alone** — non-hardening paste with no fibre, for smaller threads in good condition. **Never felt.** It is the wrong material category entirely — see Plumber's Hemp, Flax Tow, Oakum, Пакля: The Same Material Under Many Names. ## Choosing by the joint - Heating or hot water, steel or cast iron → flax and paste - Old, damaged, or over ~50 mm threads → flax and paste - Need to aim an elbow or valve after tightening → flax and paste - Small threads, cold or hot water, fast and clean → PTFE tape - New, clean, degreased metal, want mechanical strength → anaerobic sealant - Stainless, visible, or uneven threads, mess-free → wound cord - Gas → flax with a gas-rated paste, or yellow gas-grade PTFE tape, or anaerobic; never plain white water tape - **Drinking water → check the local rule first.** The permitted set differs by country: Hemp Thread Sealing and Drinking Water: Approved in Germany, Banned in the UK. One point holds across all of them: none of these seals a joint that is mechanically wrong. A sealant fills the helical gap inherent to a tapered thread; it does not rescue a cross-threaded, cracked, or badly mismatched fitting.