Match the DWR chemistry to the coating already on the fly. Silicone-based treatments bond to silicone-coated fabrics such as silnylon and silpoly; polyurethane and fluoropolymer treatments are built for PU-coated fabrics, and a mismatched pair beads up or flakes instead of soaking in and curing. If the fly's inner coating is still sound, spray-on keeps the treatment on the outer face where it belongs. Wash-in treats the entire item, interior included, and is the right tool only when both faces actually need it.
I was kneeling on a cold garage floor when that finally clicked, the fly of a two-person tent draped over a pair of sawhorses and a spray bottle in my hand. I'd treated the same fly the year before with whatever was on the shelf, and now the rain was beading in a few patches and soaking in flat everywhere else, the nylon going dark and heavy along the ridgeline where my hands had been pulling it taut.
So I flipped the fly inside out and ran a thumbnail along the inner face near the peak. Smooth. Faintly tacky. No chalk, no flakes, no peeling skin. That told me the waterproof coating underneath was fine and the finish on the outside was just tired. Two different layers on the same piece of nylon, and I'd been treating them like one thing.
That's the mistake I want to walk you through, because it costs people a whole bottle of the wrong product and an evening they don't get back.
Two layers, and only one of them is the DWR
The coating is what keeps water out. On a PU fly, that's a polyurethane layer on the back of the nylon. On a silnylon or silpoly fly, the silicone itself is the waterproof layer, which is why those tents get sealed at the seams instead of taped.
The DWR is a separate finish on the outer face. It doesn't stop water from entering the fabric so much as stop the fabric from wetting out, so the face doesn't go dark and heavy and choke off whatever breathability the tent has. When the DWR wears off, you don't usually get a leak. You get a fly that sags under a film of water and a tent that rains condensation on the inside, which is exactly the symptom people misread as a leak and try to fix with the wrong bottle.
How to tell which one you're dealing with
Flip the fly. Run your thumb across the inside surface.
Smooth and slightly grippy means the coating is intact. Powdery, flaky, or peeling in sheets means it isn't, and no DWR product on the market reattaches a delaminated coating. That's a reshell decision, not a re-treatment decision, and it's worth knowing before you spend an hour and a bottle on fabric that can't come back.
What the bottle is actually telling you
The front of the label is adjectives. The back is chemistry. I read the back first now, and I'm looking for two things: the chemistry noun, and the fabric types it says it's for.
Those two lines are the entire decision. Everything else on the bottle is a suggestion.
Spray-on versus wash-in is a coverage question
Spray-on puts the treatment on the outer face only. For a fly whose inner coating is still doing its job, that's what you want, because the thing you're replacing was only ever on the outside to begin with.
Wash-in treats the whole item, which means the interior gets treated too. On a fly with a PU inner coating, that's a real risk: the treatment can soften the coating on the inside face and leave you with a slick, slightly soapy surface, and if you've ever put a wash-in product through a tent, you know what it does to a coated bathtub floor. You sit on it, you slide, and you spend the night braced against a corner.
Wash-in has its place. Garments, packs, things where the inner face isn't a critical coated surface. On a fly, spray-on is the default, and wash-in is the exception you choose deliberately rather than the shortcut you take because it's faster.
Silicone, polyurethane, fluoropolymer
This is where most people guess, and this is where the guessing shows up as patchy, flaking results three weeks later.
Silicone DWR bonds to silicone. If the fly is silnylon or silpoly, the surface chemistry is silicone and a silicone-based treatment is the one that will actually sit down and stay. A PU-based product on that same fly has nothing to grab. It may look fine in the first five minutes, then it lifts in irregular patches, and those patches look worse than the even wetting-out you started with.
PU-coated flys are the opposite case. A polyurethane or fluoropolymer DWR is formulated for them. Fluoropolymers shed oil and water readily and hold up longer, but they're being phased out for environmental reasons, and the hydrocarbon and silicone alternatives that have replaced them generally need to be reapplied more often. That's not a defect. It's the trade you're making, and the practical version is that you'll be back out here in a season instead of two.
So the check is: what is the fabric coated with, and does the bottle name it. If the bottle names the chemistry and the fabric type, you're done deciding. If it only says something about maximum performance, put it down.
Damp or dry
Some treatments are meant for a clean, damp fabric, and some want a dry one. That line on the label governs, not any rule you picked up somewhere. I've watched someone spray a DWR onto a bone-dry fly that called for damp fabric, and the finish went on uneven and cured uneven.
Same goes for diluting. The label's stated directions are the directions. Nothing about a tent fly rewards improvising.
Before you buy anything, try heat
A tired factory DWR often comes back on its own.
Wash the fly with a residue-free technical wash first, because detergent residue, sunscreen, body oils, and woodsmoke all sit on top of the finish and mask it. No fabric softener, no bleach, no additives.
Then heat. Most tent makers put the number on the care tag, and it's usually a low tumble dry or a warm iron over a cloth. Heat drives off residual moisture and helps what's left of the finish redistribute across the fibers. I've had flies bead again after twenty minutes of that, and I've saved myself a bottle in the process.
If it beads after the heat, you're done. If it still wets out flat, now you go shopping, and now you know what chemistry you're shopping for.
Checking your own work
Mist the fly with a spray bottle and watch it. Beads that stand up and roll off mean the treatment took. A thin sheet that spreads and soaks in means it didn't, or the fabric wasn't ready for it, or the chemistry doesn't match what's underneath.
Then flip it and look at the inside again. If you sprayed, the inner face should look exactly as it did before. That's the tell that you kept the treatment where it belonged.
And keep the two layers straight in your head. A fly with a healthy coating and a dead DWR is a re-treatment. A fly with a chalky, flaking inside face is a piece of gear at the end of its life, and no bottle changes that.
