Threaded pipe joints fail for two reasons: the wrong sealant or no sealant at all. PTFE tape shreds, bunches, and leaves installer skill as the critical variable. Old pipe dope shrinks over time. Loctite 577 eliminates both problems — it's an anaerobic thread sealant that cures into a solid polymer seal inside the thread void, filling gaps completely without shredding, creeping, or hardening.
This guide covers everything you need to apply Loctite 577 correctly: what it is, how it cures, cure times, the full Loctite thread sealant comparison table, fluid and media compatibility, when you need an activator, and the mistakes that cause failures. Whether you're sealing compressed air fittings, hydraulic lines, water pipe, or gas — this is the reference guide.
What Is a Thread Sealant?
A thread sealant fills the spiral gap between mating threaded pipe connections to prevent fluid or gas leaks. Unlike a threadlocker — which locks bolts and fasteners against vibration loosening — a thread sealant is specifically engineered for pipe and fitting threads, where the goal is pressure-tight sealing rather than torque retention.
The two words look similar enough to cause real purchasing mistakes. A threadlocker (Loctite 243, 270, 277) works on cylindrical bolt threads and cures to high shear strength. A thread sealant (Loctite 577, 567, 542) works on tapered and parallel pipe threads and cures to fill the helical void, blocking fluid passage. The products are not interchangeable — using a threadlocker on pipe threads gives poor sealing; using a thread sealant on bolts gives inadequate locking strength.
Anaerobic thread sealants like Loctite 577 cure when two conditions are met: contact with metal ions (the catalyst) and exclusion of oxygen (the inhibitor is removed). Outside the joint, exposed to air, they remain liquid indefinitely — which is why excess sealant on the outside of a fitting stays wet long after the joint has cured internally. This is by design, not a failure. See the FAQ section for a full explanation of this common point of confusion.
For a broader overview of thread locking and sealing products, see our Thread Locking & Sealing Guide and Loctite Threadlocker Selection Guide.
What Is Loctite 577?
Loctite 577 is a single-component, medium-strength, thixotropic anaerobic thread sealant designed for general-purpose sealing of metal pipe threads and fittings. Developed by Henkel, it is the direct replacement for traditional methods including PTFE tape, hemp/jointing compound, and liquid pipe dopes.
Its thixotropic paste consistency is a deliberate engineering choice: it flows under the shear force of thread engagement but holds its position on vertical threads before assembly, preventing drip and run-off. This makes it far easier to control than thin liquid sealants on large-diameter or overhead fittings.
Loctite 577 is approved for industrial and process water systems, natural gas and LPG, hydraulic fluid, diesel, compressed air, and notably for hydrogen gas up to 100% (KIWA GASTEC QA AR 214), making it one of the few commercially available thread sealants cleared for hydrogen fuel systems. Note that for potable (drinking) water connections, use Loctite 55 sealing cord — it carries NSF 61 certification; the anaerobic liquid sealants including 577 do not.
| Property | Value |
|---|---|
| Type | Anaerobic, single component |
| Colour | Cream / off-white |
| Viscosity | Thixotropic paste (medium-high) |
| Strength | Medium — disassemble with hand tools |
| Operating temperature | -55°C to +150°C (short-term peak 200°C) |
| Max thread size | Up to 2-inch BSP |
| Max gap fill | 0.25–0.4 mm depending on substrate |
| Max pressure (sealed) | Up to 400 bar |
| Fixture time (steel, 22°C) | 10–60 minutes |
| Full cure time | 24 hours at 22°C |
| Potable / drinking water | Not NSF 61 certified — use Loctite 55 for drinking water |
| Gas approval | BS 6956 Type B; KIWA GASTEC QA AR 214 (incl. H₂) |
| Sizes available | 50 ml, 250 ml |
Thread Sealant vs PTFE Tape: Which Should You Use?
PTFE tape (also called Teflon tape or plumber's tape) has been the default for pipe thread sealing for decades — but its dominance rests on familiarity and low cost, not technical superiority. For professional and industrial applications, anaerobic thread sealants like Loctite 577 address every key weakness of PTFE tape.
| Factor | Loctite 577 | PTFE Tape |
|---|---|---|
| Shredding / contamination risk | None — cures solid inside joint | Can shred into valves, filters, pumps |
| Hydraulic systems | Approved ✓ | Not recommended — fragments in fluid |
| Vibration resistance | Excellent — cured polymer resists loosening | Poor — thread can back off under vibration |
| Gap filling | Up to 0.4 mm — fills worn or oversize threads | Minimal — tape deforms but does not fill |
| Application consistency | High — same result every time | Variable — depends on wrap technique and layers |
| Minor oil contamination tolerance | High — tolerates light surface contamination | Low — contamination compromises wrap adhesion |
| Disassembly | Hand tools for medium strength | Easy — unwind and re-tape |
| Potable / drinking water | Not NSF 61 certified — use Loctite 55 | Loctite 55 cord is NSF 61 certified for drinking water |
| Cost per joint | Higher | Lower |
| Learning curve | Low — apply to male thread, assemble | Low — but more variable outcomes |
When PTFE tape is the right call: low-pressure domestic water fittings where cost matters most, or when sealing plastic-to-metal thread connections (anaerobic sealants are not suitable for most plastic threads — see Common Mistakes below). For everything else — hydraulic systems, compressed air, gas lines, or any application where contamination of the downstream fluid is unacceptable — Loctite 577 is the better technical choice.
How to Apply Loctite 577
Correct application takes less than two minutes per fitting. The most common failures come from skipping the cleaning step or over-applying. Follow these six steps.
Step 1 — Clean and degrease. Remove all oil, grease, old sealant, and loose particles from both male and female threads. Use Loctite SF 7070 Cleaner & Degreaser or a similar solvent cleaner (brake cleaner works). Allow to dry fully — wet solvent residue will slow cure. This step is the one most often skipped, and the one most responsible for slow or failed cures.
Step 2 — Inspect threads. Check threads for damage, burrs, or excessive wear. Loctite 577 will fill gaps up to 0.4 mm, but damaged threads that prevent proper assembly will reduce sealing performance. Re-cut or replace fittings with significant damage.
Step 3 — Apply to male thread. Apply a 360° bead of Loctite 577 to the male thread, starting from the second or third thread (leaving the leading thread clear to prevent contamination of the downstream fluid). For coarser or larger threads, also apply to the female thread to ensure full void coverage. Force the material into the thread form — don't just coat the surface.
Step 4 — Assemble immediately. Engage and tighten the fitting using hand or wrench torque in accordance with the fitting manufacturer's specification. Loctite 577 begins curing on contact with metal, so assemble promptly. You have a working time of approximately one hour from application to make any positional adjustments — after that the cured sealant begins to resist repositioning.
Step 5 — Wipe excess. Clean any excess product from the outside of the fitting before cure. Excess sealant exposed to air will remain liquid indefinitely (this is normal — see the FAQs). Removing it now is easier than mechanical removal later.
Step 6 — Wait before pressure testing. Allow the joint to reach fixture strength before applying line pressure. On steel at 22°C this takes 10–60 minutes. For full chemical resistance and maximum pressure rating, allow 24 hours before full service loading. In cold conditions (below 10°C) or on passive metals, use Loctite SF 7649 activator — see the primer section below.
Cure Times
Loctite 577 is an anaerobic product — cure rate depends on temperature, substrate reactivity, and gap size. The table below covers typical conditions on carbon steel at 22°C. Brass cures noticeably faster; stainless steel and aluminium slower.
| Stage | Time (Steel, 22°C) | What It Means |
|---|---|---|
| Initial handling strength | 10–60 minutes | Joint holds position; not ready for pressure |
| Low-pressure service | 1–3 hours | Suitable for static, low-pressure testing |
| Full cure / full service | 24 hours | Full chemical resistance; rated pressure; full torque |
| Condition | Effect on Cure | Solution |
|---|---|---|
| Below 10°C | Significantly slower — may take 48–72 hrs for full cure | Use SF 7649 activator or bring assembly to room temp |
| Brass / copper threads | Faster — brass is highly reactive, near-instant initial seal | No change needed |
| Stainless steel | Slower — passive oxide layer reduces metal ion activity | Apply SF 7649 activator to female thread before assembly |
| Aluminium / zinc / cadmium plating | Slower — passive surfaces | Apply SF 7649 activator |
| Large gap (>0.25 mm) | Slower on outer surface of gap | Apply to both male and female threads; use activator |
| Heavy contamination | Can prevent cure or cause weak bond | Degrease thoroughly with SF 7070 before application |
Important note on exposed excess: Sealant squeezed outside the joint and exposed to air will never cure — it will remain liquid or tacky indefinitely. This is the correct behaviour of an anaerobic product, not a sign of product failure. The joint itself, where sealant is trapped between metal threads with no air, cures normally.
Loctite Thread Sealant Comparison Guide
Henkel makes five standard thread sealants in their Loctite range, each targeting a specific thread type, application, or substrate. Choosing the wrong product for the application is the most common purchasing mistake. The table below compares all five by the criteria that matter in practice.
| Product | Strength | Viscosity | Best Application | Thread Types | Temp Range | Stainless Steel | AIMS Product |
|---|---|---|---|---|---|---|---|
| Loctite 542 | Medium | Low (liquid) | Fine metric and BSP hydraulic / pneumatic instrument connections — M6 to M36 | Fine metric, small BSP | -55°C to +150°C | Slow without activator | View 542 |
| Loctite 565 | Low–Medium | Paste | General-purpose all-metal pipe fittings; low-pressure water, air, general plumbing | BSPT, NPT, metric parallel | -55°C to +150°C | Acceptable | View 565 |
| Loctite 577 | Medium | Thixotropic paste | General-purpose metal fittings; compressed air, hydraulics, water, gas, high-pressure lines up to 2" BSP / 400 bar | BSPT, NPT, BSPP, metric | -55°C to +150°C | Good (activator for fastest cure) | View 577 |
| Loctite 567 | Low–Medium | Low (liquid) | Stainless steel, copper, and passive metal fittings; where lower strength and easier disassembly are preferred | All pipe thread types | -55°C to +200°C | Excellent (engineered for SS) | View 567 |
| Loctite 569 | High | Paste | High-strength hydraulic systems; permanent or semi-permanent sealing; fittings that must not back off under extreme pressure or vibration | BSPT, NPT, metric parallel | -55°C to +150°C | Slow without activator | View 569 |
The most important distinction: if you're sealing bolt threads rather than pipe threads, you need a threadlocker (Loctite 243, 270, or similar), not a thread sealant. These are fundamentally different product types. See our Loctite Threadlocker Guide for bolt and fastener applications.
577 vs 567 — the question we get most: 577 is the higher-viscosity general workhorse for coarser BSP and NPT threads across all common industrial applications. 567 is lower viscosity, flows into finer thread forms, and is purpose-built for stainless steel and other passive metals where its PST (Pipeline Sealant Technology) chemistry provides faster, more reliable cure without an activator. If your fittings are predominantly stainless or copper, 567 is the better technical choice. If you're working across mixed metals in a general industrial or workshop environment, 577 covers more situations with one product.
Fluid and Media Compatibility
Loctite 577 has broad chemical resistance once cured, but there are application types it is not suited to. Check your specific fluid or gas against the table below before specifying.
| Fluid / Media | Loctite 577 Compatible? | Notes |
|---|---|---|
| Compressed air | ✓ Yes | Standard application; widely used in workshop and industrial systems |
| Water — industrial / process | ✓ Yes | Industrial and process water systems; not NSF 61 certified — use Loctite 55 cord for potable/drinking water lines |
| Steam (below 120°C) | ✓ Yes | Within -55°C to +150°C rating; verify system temp |
| Natural gas / LPG | ✓ Yes | BS 6956 Type B approved |
| Hydrogen gas (up to 100%) | ✓ Yes | KIWA GASTEC QA AR 214 — one of few sealants cleared for H₂ systems |
| Diesel fuel | ✓ Yes | Good fuel resistance once fully cured |
| Petrol / gasoline | ✓ Yes | Fuel resistant once cured; allow 24-hour full cure before exposure |
| Hydraulic oil (mineral) | ✓ Yes | Standard use; preferred over PTFE tape for hydraulic BSP threads |
| Synthetic hydraulic fluid | ✓ Yes | Check specific fluid data sheet for extreme chemistries |
| Coolant / antifreeze | ✓ Yes | Suitable for cooling system fittings |
| Refrigerant (R410A, R32, R134a) | ✗ No | Not rated for refrigerants — use Loctite 554 or approved refrigerant-grade sealant |
| Strong acids / oxidising agents | ✗ No | Anaerobic polymer is not acid-resistant; use PTFE or specialist chemical sealant |
| Ketones (MEK, acetone) | ✗ No | Solvents attack cured polymer |
| Chlorinated solvents | ✗ No | Not compatible — may cause seal degradation |
Passive Metals and When You Need an Activator
Anaerobic thread sealants cure through a reaction catalysed by metal ions. Active metals — iron, steel, copper, brass — release ions freely and drive a fast, complete cure. Passive metals — stainless steel, aluminium, zinc, cadmium plating, titanium — have oxide layers that slow or inhibit the metal ion release, resulting in slow or incomplete cure, particularly on larger gap sizes.
Loctite 577 is formulated to tolerate stainless steel without an activator for smaller gaps (under 0.25 mm) and at room temperature. But in practice, for reliable, predictable cure on stainless, aluminium, or plated fittings — particularly in cold conditions or where the fitting may be under pressure before 24 hours — using Loctite SF 7649 activator is the correct procedure.
Apply SF 7649 to the female thread as a thin film and allow 1–2 minutes for the solvent carrier to evaporate before applying Loctite 577 to the male thread and assembling. The activator provides the metal ion catalyst externally, accelerating cure to near the same rate as on active steel. It also overcomes cure issues in cold environments below 10°C.
Do not apply activator directly over the wet sealant — it is a pre-treatment for the mating surface, not a post-cure accelerant.
Common Mistakes to Avoid
Most Loctite 577 failures trace back to one of these six errors:
1. Skipping the cleaning step. The most common cause of slow or failed cure. Even light oil film on threads from machining or handling reduces metal ion availability. Clean with SF 7070 or a solvent cleaner and allow to fully dry before applying sealant. Wet threads or residual solvent both impair cure.
2. Using on plastic or non-metallic fittings. Anaerobic sealants are formulated for metal-to-metal thread engagement. On plastic threads — nylon, PVC, PVDF, polypropylene — Loctite 577 will not cure reliably (no metal ion catalyst) and can cause stress cracking in certain thermoplastics. For plastic-to-plastic or plastic-to-metal threads, use PTFE tape or a purpose-built plastic pipe sealant such as Loctite 5331.
3. Applying too much product. More is not better. A thin, continuous 360° bead on the male thread is all that is needed. Excess product is squeezed outside the joint on assembly and remains liquid permanently (anaerobic — exposed to air). It doesn't improve sealing and creates a mess. The actual seal is formed by the product trapped inside the thread void.
4. Pressurising before adequate cure. Applying full line pressure before the sealant has reached fixture strength forces uncured product out of the joint and can wash away the seal. On steel at 22°C, wait at least one hour before low-pressure testing and 24 hours before full-rated pressure. In cold conditions, wait longer or use activator.
5. Ignoring passive metal cure speed. Fitting stainless steel, aluminium, or plated fittings and expecting the same cure profile as carbon steel fittings is a setup for callbacks. On stainless without activator, fixture time can be several hours and full cure can take 48–72 hours. Use SF 7649 activator on passive metals as standard practice, not as an afterthought.
6. Combining with PTFE tape. A common field workaround — "belt and braces" thinking — that actually undermines both products. PTFE tape prevents the metal-to-metal contact needed for anaerobic cure. Either use 577 or use tape. Never both on the same fitting.
Removing Loctite 577
Loctite 577's medium-strength cure means disassembly is straightforward in most cases. Unlike high-strength products like Loctite 569, you generally don't need heat to break the joint.
Before full cure (within 24 hours): Disassemble with standard hand tools or a wrench. The partially cured polymer breaks cleanly. Clean threads with a solvent or wire brush before re-sealing.
After full cure (24+ hours): Apply a standard pipe wrench or adjustable spanner. Medium-strength cured product will break free with normal torque. For stubborn joints or in situations where you need to avoid thread damage (e.g., brass or aluminium fittings), apply localised heat with a heat gun or small propane torch to 150–200°C. This softens the polymer and allows disassembly with minimal torque.
Cleaning after removal: Wire brush, nylon brush, or a clean rag with solvent (acetone or SF 7070) removes residual product from threads. Allow to dry before applying fresh sealant. Do not re-use degraded or contaminated sealant from the old joint — apply fresh product.
Frequently Asked Questions
What is Loctite 577 used for?
Loctite 577 is used for sealing metal pipe threads and fittings against leaks in compressed air, hydraulic, water, gas, diesel, and steam systems. It replaces PTFE tape and traditional pipe dope by curing into a solid polymer seal inside the thread void. It is not a threadlocker and should not be used on bolt or fastener threads.
How long does Loctite 577 take to cure?
On carbon steel at 22°C: initial handling strength in 10–60 minutes, suitable for low-pressure testing after 1–3 hours, full cure and full rated pressure after 24 hours. Brass cures faster; stainless steel and aluminium slower. In cold conditions (below 10°C) or on passive metals, use Loctite SF 7649 activator to achieve practical cure times.
What is the difference between Loctite 577 and 567?
577 is a thixotropic paste suited to coarser BSP and NPT threads in general industrial applications across all common metals. It is the standard workhorse for compressed air, water, hydraulic, and gas fittings. 567 is a lower-viscosity liquid that wicks into finer thread forms and is purpose-built for stainless steel and passive metals, where its PST chemistry delivers reliable cure without requiring an activator. If your installation is predominantly stainless steel or copper fittings, 567 is the better technical choice. For mixed-metal environments with larger threads, 577 covers more ground with one product.
Is Loctite 577 suitable for fuel, gas, and water lines?
Yes for industrial applications. Loctite 577 is approved for natural gas and LPG (BS 6956 Type B), hydrogen gas up to 100% (KIWA GASTEC QA AR 214), diesel, petrol, hydraulic oil, and industrial water and process water systems. It is not NSF 61 certified — for potable (drinking) water connections, use Loctite 55 sealing cord. It is also not suitable for refrigerants (R410A, R32, R134a) — use Loctite 554 for refrigerant line fittings.
Is Loctite 577 removable?
Yes. Loctite 577 is medium strength and designed for disassembly with standard hand tools in most cases. For very tight or long-cure joints, applying localised heat (150–200°C) softens the polymer and makes disassembly straightforward. This is distinct from high-strength thread sealants like Loctite 569, which require more effort to remove.
What is the difference between a thread sealant and a threadlocker?
A thread sealant (Loctite 577, 567, 542) seals pipe and fitting threads against fluid or gas leakage. A threadlocker (Loctite 243, 270, 277) locks bolts and fasteners against vibration-induced loosening. The chemistry is similar but the gap fill, strength, and application are different. Using the wrong type for the wrong application gives poor results — do not substitute one for the other.
Is Loctite 577 better than PTFE tape?
For most industrial, hydraulic, and professional applications: yes. Loctite 577 fills thread gaps completely, eliminates shredding contamination risk, tolerates minor oil contamination, and provides consistent results regardless of installer technique. PTFE tape is banned from hydraulic systems precisely because fragments cause damage to valves and pumps. For simple domestic water fittings where cost is the primary concern, PTFE tape remains practical. For anything involving hydraulic fluid, gas, or high pressure, use Loctite 577.
Can Loctite 577 be used on stainless steel?
Yes, with some qualification. Loctite 577 will cure on stainless steel, but more slowly than on carbon steel, because stainless steel's passive oxide layer reduces metal ion availability for the anaerobic cure reaction. For smaller gaps and normal temperatures, it cures adequately without an activator within 24 hours. For reliable, fast cure on stainless — particularly in cold conditions or where early pressure loading is required — apply Loctite SF 7649 activator to the female thread before assembly.
Can Loctite 577 be used on plastic fittings?
No. Anaerobic thread sealants require metal-to-metal thread contact to cure — plastic provides no metal ion catalyst. Loctite 577 will not cure reliably on plastic threads and can cause stress cracking in certain thermoplastics (particularly PVC, CPVC, and ABS). For plastic-to-plastic or plastic-to-metal connections, use PTFE tape or Loctite 5331 Plastic Pipe Sealant, which is specifically formulated for plastic thread systems.
Why is excess Loctite 577 still wet after 24 hours?
This is normal. Loctite 577 is an anaerobic product — it only cures when oxygen is excluded. Inside the joint, where sealant is trapped between metal thread surfaces with no air contact, curing proceeds normally. Outside the joint, exposed to air, the sealant remains liquid indefinitely. The wet exterior is not a sign of failure — the joint itself is cured. Wipe off the excess before it hardens into a difficult-to-remove skin.
When do I need an activator with Loctite 577?
Use Loctite SF 7649 activator when working with passive metals (stainless steel, aluminium, zinc, plated surfaces), in cold conditions below 10°C, when you need full cure in less than 24 hours, or when gap sizes exceed 0.25 mm on passive surfaces. Apply a thin film of activator to the female thread, allow the solvent carrier to evaporate for 1–2 minutes, then apply Loctite 577 to the male thread as normal and assemble. Do not apply activator over wet sealant.
What Loctite thread sealant should I use for fine hydraulic threads?
For fine metric or BSP hydraulic and pneumatic instrument connections (M6–M36 or small-bore hydraulic block fittings), use Loctite 542 — its low viscosity wicks cleanly into fine thread forms without over-filling. Loctite 577 is higher viscosity and better suited to coarser BSP and NPT threads up to 2-inch diameter. For general hydraulic fittings (3/8" BSP and above), 577 is the standard choice.
Shop Loctite Thread Sealants
AIMS Industrial stocks the full Loctite anaerobic thread sealant range in 50 ml and 250 ml sizes, including 542, 565, 567, 577, and 569 — with same-day despatch from our Milperra warehouse. If you're replacing PTFE tape or jointing compound across a facility, the Loctite thread sealant range covers every thread type, pressure rating, and substrate combination you'll encounter.
Need help selecting the right product for your system? Our technical team at AIMS Industrial can advise on product selection for specific fluids, pressures, and substrate combinations — contact us directly or visit our Loctite Product Guide for further guidance.
For related sealing applications, see our guides on RTV Silicone & Gasket Maker selection and Loctite 401 Instant Adhesive.