How to Waterproof a Tent Fly and Seams: Silicone vs PU Coating Guide

Introduction

A tent fly loses its waterproofing through two mechanisms: the coating degrades, and the seam tape or sealant fails. These happen on different timelines. Seam tape usually fails first, sometimes within two to three years, especially on tents stored in hot attics or car trunks where sustained heat softens and deforms the adhesive. The coating on the fly fabric itself degrades more slowly but eventually hydrolyzes (in the case of polyurethane) or wears thin from abrasion and UV exposure.

The waterproofing approach you use depends entirely on what your tent fly is made of. Applying the wrong treatment to the wrong coating does nothing at best and at worst creates a sticky, peeling mess that cannot be undone.

This guide covers the three most common tent fly materials: PU-coated nylon or polyester, silicone-coated nylon (silnylon), and the two-sided hybrid coating used on many modern tents. It also covers seam sealing for each type.

Identifying Your Tent's Coating Type

Before doing anything, identify the coating. Here is how:

PU coating: Feels slightly rubbery or tacky to the touch. The interior side of the fly has a shiny, uniform plastic-like appearance. When you rub it with a damp finger, it does not feel greasy. PU coatings are typically on the inside of the fly (facing the tent body). Examples: REI Co-op tents, Big Agnes C Bar, budget backpacking tents.

Silicone coating (silnylon): Feels slippery and has a slightly translucent quality when held up to light. Water beads up aggressively even when the coating is old because silicone is inherently hydrophobic. Silnylon has a softer hand feel and crinkles less than PU-coated fabric. Examples: MSR Hubba series (fly), Mountain Hardwear tents, ultralight shelters from Zpacks and Six Moon Designs.

Hybrid (sil/PU): Many modern tents, including the MSR Hubba Hubba and Big Agnes Copper Spur, use a silicone coating on the outside of the fly (for water repellency and UV resistance) and a PU coating on the inside (to allow seam tape to adhere). This dual coating combines the durability of silicone with the convenience of taped seams.

To test which side is which: rub a corner of the fly with a damp finger. The silicone side feels slick and the water beads instantly. The PU side feels slightly rubbery and water sits without beading.

Waterproofing PU-Coated Tent Flys

PU coatings fail through hydrolysis. Polyurethane reacts with water over time and the long polymer chains break into shorter segments. The coating loses its mechanical integrity and eventually turns sticky, then peels away in flakes. This process is accelerated by storing the tent damp or in hot conditions.

If the PU coating is peeling: You are looking at a re-coating project, not a waterproofing project. Gear Aid Seam Grip or McNett Tent Sure can be thinned and painted over the peeling PU layer to create a new waterproof barrier. This is labor-intensive. For a tent with widespread peeling (more than 30 percent of the fly area), replacement is the practical choice. The cost of re-coating supplies and the time investment exceed the value of most PU-coated tents under 300 dollars.

If the PU coating is intact but the fly wets through: The DWR on the exterior face has worn off, and the underlying PU is still functional. Apply an exterior DWR treatment:

• Nikwax Tent and Gear SolarProof: Spray-on treatment that provides water repellency and UV protection. Spray the exterior of the pitched tent until evenly coated. Wipe off drips. Let dry completely before packing.

• Grangers Fabsil Universal Protector: Silicone-based spray. Highly effective for waterproofing the exterior face. Use in a well-ventilated area; the solvent odor is strong. Fabsil is available in spray and liquid (paint-on) formats.

• Gear Aid ReviveX Tent Waterproofer: Spray-on polymer similar to DWR for clothing. Less durable than Fabsil but easier to apply and lower odor.

Applied to the exterior only, these treatments restore water beading on the face fabric. The PU coating on the inside continues to do the actual waterproofing.

Waterproofing Silicone-Coated (Silnylon) Tent Flys

Silnylon does not fail through hydrolysis the way PU does. Silicone is chemically stable for decades. However, silicone coatings can abrade away from the fabric fibers through pack friction and folding wear. When a silnylon fly starts to mist through (a fine water spray comes through the fabric under pressure), the silicone coating has worn thin.

The correct product: You cannot use PU-based seam sealers or DWR sprays on silnylon. Nothing water-based will bond. You need a solvent-based silicone treatment.

• Gear Aid SilNet: A silicone-based liquid sealant specifically for silnylon. Mix SilNet with mineral spirits (odorless type, from a hardware store) at a ratio of roughly 1 part SilNet to 3 to 4 parts mineral spirits. The goal is a thin, paint-like consistency that can be brushed on.

• McNett Sil-Net: Same product, older branding.

• DIY alternative: Clear 100 percent silicone caulk (GE Silicone II or similar) mixed with mineral spirits. It is the same active ingredient. The ratio for the DIY version is roughly 1:4 silicone to mineral spirits, but test consistency on a scrap area first. This is more work and more odor but costs about 80 percent less than branded products by volume.

Application process: Pitch the tent fly taut. Brush or roll the thinned silicone mixture onto the exterior of the fly. Work in sections. The mineral spirits will flash off (evaporate) within 10 to 15 minutes, leaving a thin, even silicone layer bonded to the fabric. Apply a second coat if the fly was heavily misting.

The fly will have a slight silicone smell for a few days after treatment. Pitch it outdoors for a day before packing it for storage.

Important: Do not apply silicone-based treatments to the inside of a hybrid-coated tent whose interior is PU. Silicone will not bond to PU and will create a sticky layer that cannot be removed.

Seam Sealing Your Tent

Seams are the number one leak point on any tent. Every needle hole in the fabric is a potential water entry point. On factory-sealed tents, seam tape covers these holes. On silnylon tents, a liquid seam sealant must be applied.

Seam Sealing a New Silnylon Tent

Many silnylon tents from cottage manufacturers (TarpTent, Six Moon Designs, Mountain Laurel Designs) arrive unsealed. You must seal the seams yourself before using the tent.

1. Pitch the tent or lay the fly out flat with the seams accessible.
2. Clean the seam area with isopropyl alcohol and a lint-free cloth. Silicone rejects anything but the cleanest surface.
3. Apply Gear Aid SilNet or the DIY silicone-mineral spirits mixture in a thin bead along the seam, covering each stitch line.
4. Use a small brush or the tip of the tube to work the sealant into the stitching holes.
5. Let cure for 24 hours in a ventilated area. Silicone sealants cure by absorbing moisture from the air and releasing acetic acid (vinegar smell). The full cure time is necessary.
6. Test the seam with a hose or in the next rain. Any leaking needle holes need a spot application.

Repairing Factory Seam Tape

On PU-coated tents with factory taped seams, the tape eventually peels, cracks, or becomes sticky. When this happens:

1. Do not attempt to re-stick the peeling tape. Once thermoplastic seam tape has lifted, the heat-activated adhesive has degraded and will not re-bond.
2. Remove the loose tape gently. Do not pull aggressively; you will tear the PU coating off with the tape. Use a seam ripper or a sharp knife to carefully cut along the edge of the tape where it is still adhered, leaving the good section intact.
3. Clean the exposed seam area with isopropyl alcohol.
4. Apply a thin bead of Gear Aid Seam Grip (the original formula, not SilNet) along the exposed seam line. Seam Grip is a polyurethane-based sealant that bonds to PU coatings.
5. Spread the sealant evenly with a small brush or a clean fingertip (wear a nitrile glove). Aim for a 5 to 8 mm wide strip covering the entire stitch line.
6. Cure for 12 to 24 hours.

Seam Grip is also used for repairing small holes and tears in PU-coated fabrics. It can be applied as a thin layer or, for larger repairs, used to bond a patch of compatible fabric over the damage.

Seam Sealing Hybrid (Sil/PU) Tents

For tents with a silicone exterior and PU interior, the factory tape is on the inside (PU side). If this tape fails, treat the inside with Seam Grip (PU-compatible). The silicone exterior seams do not typically need sealing because the PU layer on the inside handles the waterproofing, but if you see leaking through the exterior seam line, apply SilNet to the exterior as well.

When to Re-Coat vs. When to Replace

A tent fly is worth the effort of re-waterproofing if the coating is intact but the water repellency is gone. A tent that needs full re-coating (where the original coating has peeled off) is a more complex calculation:

• If the tent is under 3 years old and moderately priced, re-coating makes sense.
• If the tent is over 5 years old and the coating has failed, the fabric integrity is also likely in decline. Nylon loses roughly 5 percent of its tensile strength per year from UV exposure, even with UV inhibitors in the coating. A 10-year-old tent with peeling PU may tear easily and is not worth the re-coating investment.
• DCF (Dyneema Composite Fabric) tents do not have traditional coatings and cannot be re-waterproofed in the field. Their waterproofing is inherent to the material. If a DCF tent leaks, the leak is at a seam or through a puncture. Patch with DCF tape (Zpacks and DutchWare sell compatible tapes).

Conclusion

Tent fly waterproofing is a two-part task: re-treating the fabric, and sealing the seams. The critical step is identifying whether your fly uses PU, silicone, or a hybrid coating, and using the correct product for that coating. PU needs polyurethane-based sealants (Seam Grip) and DWR sprays. Silicone needs silicone-based sealants (SilNet) thinned with mineral spirits. Using the wrong product wastes time, wastes money, and in some cases (silicone on PU, or vice versa) creates a mess that is harder to fix than the original problem.