PTFE, Hemp, Liquid Sealant, or Solvent Cement: The Trade Guide to Plumbing Sealing Materials

PTFE, Hemp, Liquid Sealant, or Solvent Cement: The Trade Guide to Plumbing Sealing Materials

Ask a plumber what they use to seal a threaded joint and you will get a different answer depending on who trained them and when. Hemp and Boss White, PTFE tape, liquid PTFE sealant — they all work, but they are not all right for every application. Using the wrong material leads to weeping joints, failed potable water compliance checks, or joints that cannot be re-made cleanly. This guide covers the main sealing materials, where each one works, and where it doesn't.

PTFE Tape

PTFE (polytetrafluoroethylene) tape, also called plumber's tape or thread seal tape, is the most widely used jointing material for male BSP (British Standard Pipe) tapered threads. It works by filling the voids between thread flanks as the joint is tightened, creating a pressure-tight seal.

When to use: Tapered BSP male threads on water supply, central heating, and gas pipework (see colour note below). Compression fitting tail connections on appliances. Valve spindle glands (wrapped around the spindle to stop drips).

When NOT to use: Parallel (straight) BSP threads — these are face-seal joints that rely on a compression washer or O-ring for sealing, not thread engagement. Wrapping PTFE on a parallel thread achieves nothing and can prevent the joint from tightening fully. Check whether your fitting uses a tapered or parallel thread before applying PTFE.

Colour coding:

• White PTFE — water supply and heating. Standard grade.
• Yellow PTFE — gas service (BS EN 751-3 compliant). Higher density, greater thickness. Never use white PTFE on gas — it is not approved for gas service.

Application: Wrap clockwise (looking at the male thread from the end) — the same direction as tightening. Minimum 3 turns for water, 5+ turns for gas or higher pressure connections. More wraps = tighter joint, but do not over-wrap to the point where the fitting cannot be tightened fully.

Liquid PTFE Pipe Sealant

Liquid PTFE sealants are a more modern alternative to tape and hemp, combining the sealing properties of PTFE with a liquid carrier that flows into thread voids and sets to a flexible seal. Products like GFS Pipe Sealant are WRAS approved for potable water supply, which makes them the correct choice for water supply threaded connections where tape can sometimes work loose on high-vibration or high-cycle applications.

Advantages over tape: applies in seconds with a brush or nozzle, fills voids more completely in worn or damaged threads, and holds pressure better on low-quality castings where thread form is inconsistent.

The GFS Liquid PTFE Pipe Sealant we stock is WRAS approved for cold and hot water supply up to 95°C. Apply a bead around the male thread, coat evenly, and assemble immediately — it does not need to cure before the joint is pressurised.

Hemp and Jointing Compound

The traditional method: plumber's hemp fibres wound onto the male thread, then coated with a jointing compound (Boss White being the most recognised UK brand). The hemp provides mechanical bulk that fills the thread, and the compound lubricates during assembly and seals against weeping.

When to use: Tapered BSP threads on iron and steel fittings where the thread form is coarser and PTFE tape alone may not provide sufficient sealing. Preferred by many experienced plumbers on large-diameter threaded joints (¾" BSP and above) where tape can be fiddly. Also preferred on radiator connections, boiler flow/return connections, and flanged iron pipework.

Application: Wind hemp clockwise into the thread valleys, building up a uniform layer without too much bulk. Apply jointing compound over the hemp — a thin coat, not a thick layer. Do not use so much compound that it enters the system when the joint is assembled (excess compound in a heating system contributes to sludge).

Hemp and compound is demountable: you can unscrew the joint and re-make it. Unlike thread sealant compounds that harden permanently, Boss White stays workable for years.

Solvent Cement for Waste Pipe

Solvent cement is not a sealant in the conventional sense — it is an adhesive that chemically welds plastic pipe to plastic fittings. Used on ABS and uPVC waste pipes and drain pipes (32mm, 40mm, 50mm), it dissolves the pipe and fitting surfaces and fuses them together as it cures.

Key requirements:

• Clean and dry surfaces — solvent cement will not bond to wet, dirty, or contaminated surfaces. Wipe with acetone or the manufacturer's cleaner first.
• Correct cement for the material — ABS cement on ABS pipe, PVC cement on PVC. They are not interchangeable in all cases.
• Assembly within 30 seconds — the cement activates fast. Apply to both surfaces (pipe end and socket), push together with a quarter-turn, hold for 15 seconds. Do not disassemble once joined.
• Cure time before pressure — 15 minutes before running water, 2 hours before full pressure. Cold weather extends cure time.

We stock the ProXCare 125ml Solvent Cement and Black Swan PVC Cement 118ml for waste and drain pipe jointing. Do not use solvent cement on potable water pipework — it is not WRAS approved for water supply.

Solder for Copper Pipe

End-feed and solder ring copper fittings require solder to complete the joint. Current UK regulations require lead-free solder on all potable water pipework (BS EN 29453 compliant). Lead solder is still legal for use on central heating systems only.

For most copper soldering work on domestic installations, use lead-free solder wire with a separate flux. Clean the pipe end and fitting with emery cloth, apply flux, assemble, heat evenly with a torch, and feed solder into the joint gap. The solder flows by capillary action when the correct temperature is reached (the flux helps judge this — it should be bubbling). Do not overheat; copper discolouration to purple-black indicates the flux has burned off and the joint will fail.

• AIM Lead-Free Solder 500g — potable water and general copper work
• AIM Leaded Solder 500g — heating systems only

Quick Reference: Which Material for Which Job

In stock at 24 Western Avenue, Acton W3 7TZ — call 020 8702 8080.

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