Seam Sealing Explained: What It Is and When It Matters

A seam is a row of needle holes

Every piece of sewn gear has a secret hiding in plain sight. A sewing machine joins two panels of fabric by driving a needle through both of them, over and over, hundreds of times per seam and thousands of times per garment. Each stitch is a hole punched clean through the cloth. The thread fills most of each hole, but not all of it, and thread itself can wick water along its length like a lamp wick. So a "waterproof" fabric sewn into a finished shape is a waterproof fabric interrupted by tidy rows of small punctures.

Seam sealing is the family of fixes for those punctures. On most rain gear built from PU-coated or membrane-laminate fabrics, the fix happens at the factory: a heat-activated waterproof tape is bonded over every stitch line on the inside of the garment. When a jacket hangtag says "fully taped seams," that is what it means. The tape bridges the needle holes so water cannot pass through them or wick along the thread.

The other fix is liquid. On silicone-treated fabrics, the seam gets painted instead of taped: a thin coat of liquid sealant is brushed along the stitch line, soaks into the holes and the thread, and cures into a flexible seal. This is the method the tent and tarp world has used for decades, and it is the one that matters for the fabric our shell is built from.

A seam is a line of needle holes through the fabric, and seam sealing, whether tape or liquid, is simply how those holes get closed.

When sealing genuinely matters, and when it is at the margin

Water does not pour through needle holes on its own. It has to be pushed, and it has to be given time. The two variables that decide whether unsealed seams are a real problem or a footnote are pressure and duration, and it is worth being concrete about where each one shows up.

Shelters are the clearest case. A tent fly pitched taut in standing rain holds a film of water against its seams all night. A tent floor takes the full pressure of a body, a sleeping pad, and everything in the pack, pressed down onto wet ground for eight hours. A dry bag gets submerged outright. These are static, loaded, hours-long exposures, and unsealed seams will seep in every one of them. This is why seam sealing is non-negotiable on shelters and dry bags.

Garments have their own pressure points. A human rain shell gets taped at the factory largely because of what happens under a pack: shoulder straps and a hipbelt press soaked fabric hard against the body for a full day of walking, and kneeling or sitting does the same at the knees and seat. Under that kind of clamped, sustained load, needle holes become a real path, and taping earns its cost.

Now consider a dog moving down a trail. The shell drapes over an animal that is constantly in motion. Water hits the fabric and runs off. Nothing straps the fabric against the body, nothing pools on it, and nothing holds it under pressure for hours. Seam seepage under that kind of use is a marginal path compared to the coverage the fabric itself provides. Pressure and time are what turn needle holes into leaks, and a moving dog shedding weather gives water very little of either.

Seam sealing matters where water sits under load for hours; on a moving animal that sheds weather, it lives at the margin.

Why sil-fabrics are a special case

The Traverse Shell is built from silpoly, a polyester ripstop impregnated with silicone. We wrote a full piece on what silpoly is and why we chose it, but the short version matters here: the silicone is not a coating sitting on the surface, it is cured into the fibers themselves, and it is the reason the fabric is waterproof and windproof at such a low weight.

That same silicone creates the special case. Cured silicone is famously non-stick, and almost nothing bonds to it except more silicone. Factory seam tape, the kind that works on PU-coated and membrane fabrics, will not hold on silicone-treated cloth. This is not a corner a manufacturer can choose to cut or not cut. It is a material property, and it is why silnylon and silpoly tents, including shelters trusted on thru-hikes measured in thousands of miles, have shipped with unsealed seams for decades.

The accepted answer in that world is liquid silicone seam sealer, such as Gear Aid Seam Grip+SIL, formerly sold as SilNet. It is silicone in a tube, thinned enough to brush along a stitch line, and because it is silicone it bonds to the fabric the way tape never will. Owners of sil-fabric shelters apply it themselves as a matter of routine, and the seal it cures into is flexible and long-lived.

Tape does not bond to silicone-treated fabric; the fix for sil-fabric seams is liquid silicone, applied by whoever owns the gear.

The Traverse Shell, stated plainly

The Traverse Shell is not seam-sealed. We would rather put that sentence at the top of the section than bury it in a footnote or let a spec sheet imply something the gear does not do. It is the same reasoning behind our refusal to publish a hydrostatic-head number we cannot stand behind: we would rather tell you plainly what a layer is built to do than let an impressive-sounding claim write a check the gear has to cash in weather.

Here is what the shell is built to do. The fabric is a waterproof, windproof, silicone-impregnated silpoly ripstop, and its job is shedding wind and sustained precipitation on a moving dog. In that use, as laid out above, the seams are a marginal path: water runs off an animal in motion before pressure and time can push it through stitch holes. Taping the seams was never on the table, because tape does not bond to silicone-treated fabric. What remains is a shell that does its actual job at a weight that earns its place in the pack.

And here is what it is not built to do. It is not a submersible dry bag, and it is not a shelter pitched taut in standing rain all night. If your trip plan includes hours of static, loaded exposure, that is a job for a tent, not for any garment. When you are comparing shells, ours or anyone's, this is exactly the kind of question worth asking out loud, and it is why we wrote guides on how to read a spec sheet and how to choose an outer layer in the first place.

An unsealed seam is not a defect we are confessing to. It is a design decision we are explaining, the same way a tarp maker explains why the shelter ships with a tube of sealant instead of taped seams. What we will not do is print "waterproof fabric" on a product page and stay quiet about the seams, because silence in that spot invites you to assume something the gear was never built to do. You should know exactly where the limits of a layer sit before you are standing on the wrong side of them.

The shell sheds wind and sustained precipitation on a moving dog; it is not seam-sealed, and we would rather say so than let you find out in the field.

How to seal it yourself, if your miles call for it

Some use cases sit past the margin. If your seasons include canoe trips, long stationary days in steady rain, or a dog that spends real time lying on soaked ground in weather, sealing the seams yourself is a legitimate move, and on a sil-fabric it is a genuinely easy one. This is the same field-maintenance ethos as renewing a DWR finish on layers that have one: a small evening of work that extends what a piece of gear can honestly do. The shell itself has no DWR to renew, since its silicone is impregnated into the fabric for good, so this is the one maintenance job it will ever ask of you, and only if your conditions demand it.

The method comes straight from the tent world. Clean the layer and let it dry completely, because sealant will not bond through grime. Lay it flat with the seams accessible, and brush a thin, continuous coat of silicone seam sealer along each stitch line on the side that faces the weather, working the sealant into the stitching itself so it penetrates the needle holes rather than just bridging them. Thin is the discipline here. A thin coat flexes with the fabric and lasts; a thick bead stays tacky, collects grit, and peels.

Then leave it alone. Work in a ventilated space, follow the cure time on the tube, and give it overnight before you pack or fold the layer. Once cured, the seal is flexible, washable, and effectively permanent, the same treatment protecting silpoly tarps through entire thru-hikes. An hour of brushwork and a night of curing, and the one marginal path through the shell is closed.

A thin coat of silicone sealer along the stitch lines, cured overnight, turns an unsealed seam into a sealed one, and it is a skill worth having in any sil-fabric owner's kit.

Adventure On.