Sealing a joint depends on what you’re connecting: threaded pipes, plastic fittings, metal flanges, or expansion gaps in concrete and masonry. Each type of joint calls for a different sealant and technique, but the core principle is the same. You need to fill every microscopic gap between two surfaces so that liquid, gas, or air cannot pass through. Here’s how to do it right for the most common joint types.
Threaded Pipe Joints
Threaded connections on water lines, gas lines, and air systems are never perfectly smooth. Even tightly screwed threads have tiny spiral gaps that will leak without a sealant. You have two main options: tape or pipe joint compound.
PTFE tape (commonly called Teflon tape) is a thin, stretchy film you wrap around the male threads before assembly. It fills those micro-gaps and also lubricates the threads so you can tighten the connection further without binding. Wrap the tape in the same direction the fitting will screw on, typically clockwise when you’re looking at the end of the pipe. Use three to five tight, overlapping passes. PTFE tape is compatible with all pipe materials, resists heat and chemicals, and works well on water lines, air lines, and compression fittings.
Pipe joint compound (pipe dope) is a thick paste you brush onto threads. It’s sold in small cans with a built-in applicator brush. Because it’s thicker than tape, it fills larger imperfections and creates a more forgiving seal on rough or slightly damaged threads. That thickness makes it the preferred choice for gas lines, where even a pinhole leak is dangerous. The tradeoff: some formulations can react with soft plastics and corrode certain fittings, so check compatibility before applying it to plastic pipe.
For the strongest seal on threaded connections, many plumbers use both: a few wraps of PTFE tape followed by a thin coat of pipe dope over the tape. This gives you the chemical compatibility of the tape with the gap-filling power of the compound.
Solvent-Welded Plastic Joints
PVC, CPVC, and ABS pipes don’t use mechanical threads or compression fittings for permanent connections. Instead, they rely on solvent cement, which chemically fuses the pipe and fitting into a single piece of plastic. This isn’t glue in the traditional sense. The solvent softens the surface of both the pipe and the fitting, making them semi-fluid. When you push them together while still wet, the softened surfaces merge. As the solvent evaporates, the material re-hardens and the two pieces become one continuous wall of plastic with no seam.
The process has a narrow window. You need to work quickly because the cement must still be wet and fluid when you assemble the joint. If it starts to dry before assembly, the surfaces won’t fuse properly. Here’s the sequence:
- Dry-fit first. Push the pipe into the fitting without cement to check the fit. The pipe should slide in one-third to two-thirds of the way before friction stops it.
- Clean and prime. Wipe both surfaces with a clean rag to remove dirt and moisture. For PVC and CPVC, apply primer to the outside of the pipe and the inside of the fitting socket. Primer softens the surface and prepares it for the cement.
- Apply cement generously. Brush a heavy, even coat on the pipe end and a thinner coat inside the fitting. Enough cement must be present to fill the gap between pipe and fitting in the looser areas of the joint.
- Assemble immediately. Push the pipe into the fitting with a quarter-turn twist and hold it for 15 to 30 seconds to prevent the pipe from backing out. A ring of excess cement around the joint edge is normal and means you used enough.
Joint strength builds as the solvent evaporates. Most manufacturers recommend waiting at least two hours before handling the connection and 24 hours before pressurizing the system with water.
Silicone and Polyurethane Sealant Joints
Where two rigid surfaces meet but can’t be welded or threaded together, flexible sealants bridge the gap. Silicone and polyurethane sealants are the two most common choices, and they cure through different chemical processes that affect how and where you use them.
Silicone sealants (RTV, or room-temperature vulcanizing) cure by reacting with moisture in the air. As humidity triggers the chemical reaction, the silicone forms a flexible, rubbery network of bonds. A small amount of byproduct (often acetic acid, which you can smell as a vinegar-like odor) releases during curing, so you need ventilation. Silicone is excellent for joints exposed to heat, water, or chemicals, like around plumbing fixtures, HVAC connections, or engine components.
Polyurethane sealants also cure with moisture but tend to be tougher and more paintable than silicone. They cure slowly: at 75°F and 50% relative humidity, expect a surface skin to form in about 30 hours, and the sealant to cure roughly 1/16 inch deeper per day after that. A half-inch-deep joint could take over a week to fully cure in moderate conditions. Higher humidity speeds things up; dry conditions slow them down significantly.
For either type, surface preparation determines whether the seal holds or peels away. Clean the joint surfaces of grease, oil, dust, and loose material before applying sealant. For porous surfaces like concrete or masonry, a primer may be needed. Apply the sealant in a continuous bead, then tool it (smooth it with a wet finger or a caulking tool) to press it firmly against both surfaces and eliminate air pockets.
Anaerobic Sealants for Metal Joints
Anaerobic sealants work on a different principle: they cure in the absence of air, specifically when squeezed between two close-fitting metal surfaces. This makes them ideal for sealing flanges, threaded bolts, and hydraulic fittings where a tight metal-to-metal fit is already present.
The metal itself plays a role in how fast the sealant cures. Active metals like brass, copper, and carbon steel trigger fast polymerization. Less reactive metals like aluminum, stainless steel, cast iron, and electroplated surfaces cure much more slowly and often need an activator sprayed on the surface beforehand to jump-start the chemical reaction.
Surface cleanliness matters more here than with other sealants. Grease, oil, and machining residue will block the chemical reaction. Clean both surfaces with a degreasing solvent or a water-based cleaner, and for heavily contaminated parts, mechanical methods like wire brushing or light sandblasting will help. Apply the sealant to one surface, assemble, and let it cure undisturbed. Most anaerobic sealants reach handling strength in a few hours and full strength in 24 hours.
Expansion Joints in Construction
Buildings, sidewalks, and bridge decks contain deliberate gaps between sections of concrete, masonry, or cladding. These expansion joints absorb the movement caused by temperature changes, settling, and seismic activity. Sealing them keeps water, debris, and air from penetrating the structure.
The critical factor for expansion joints is movement capability, which is rated as a percentage of the joint width under ASTM C920. A sealant rated at Class 25, for example, can handle the joint stretching or compressing by up to 25% of its original width without losing adhesion. Wider joints or joints in climates with extreme temperature swings need higher-rated sealants. Choosing a sealant with too little movement capability is one of the most common reasons expansion joint seals crack and fail within a few years.
To seal an expansion joint, first remove any old sealant and clean the joint walls. Insert a foam backer rod to control the depth of the sealant. The ideal sealant depth is typically half the joint width. Apply the sealant over the backer rod, filling to the surface, and tool it smooth. The backer rod prevents the sealant from bonding to the bottom of the joint, which would restrict its ability to stretch. A properly installed expansion joint seal bonds only to the two side walls, allowing it to flex like a rubber band as the joint moves.
Why Joint Seals Fail
Most seal failures come down to three causes: poor surface preparation, choosing the wrong sealant for the application, or not allowing enough cure time before putting the joint into service.
Temperature cycling is another common culprit. Different materials expand at very different rates when heated. Copper expands at roughly 16 to 17 millionths of a meter per meter for every degree Celsius of temperature change. PVC expands several times faster. When two different materials meet at a joint, or when a single material undergoes large temperature swings, the sealant must absorb that movement without tearing or losing adhesion. Rigid sealants in high-movement joints will crack. Flexible sealants on high-pressure gas lines may not hold. Matching the sealant to the specific demands of the joint, including pressure, temperature range, chemical exposure, and movement, is the single most important step in getting a lasting seal.