Minor damage to a fly line’s coating—such as a 1/8-inch gap exposing the core—should be repaired to prevent water from soaking into the core, which would cause the line to sink or cast poorly. Below are two approaches: a quick, temporary fix to keep you fishing in the short term, and a long-term repair to restore functionality and durability. We also list some recommended products and kits for these repairs.

Temporary Fix

For a fast fix, you can seal the exposed section with a quick-setting adhesive. This will waterproof the core and hold the coating together long enough for a day’s fishing (though it is ultimately a temporary solution). Flexible adhesives or glues—UV-cure resin like Loon UV Knot Sense, or a gel-type flexible super glue like Zap-A-Gap or Loctite 414—can patch a small nick in the coating. The UV resin has the advantage of curing in seconds under sunlight/UV light. Here’s how to apply a quick fix:

1. Clean and Dry the Area: Rinse the damaged section of line with fresh water and dry it thoroughly. Removing dirt ensures better glue adhesion.

2. Expose the Gap: Gently flex or fold the line at the gap so that the coating opens up and the **core is exposed** (this makes it easier to get adhesive inside).

3. Apply Adhesive: Using a toothpick or needle, apply a small drop of **adhesive** into the gap. Make sure to coat the **ends of the plastic coating and the core** itself. For example, put a drop of UV resin or flexible super glue on the tip of the toothpick and smear it into the crack.

4. Close and Secure: Straighten the line to close the gap, squeezing the coating back together. Wipe off any excess glue that oozes out, and use a damp finger or the toothpick to smooth it along the line. Rotate the line 90° and repeat the process if the gap wasn’t fully filled on all sides. The goal is a thin, even coat of adhesive sealing the break all around.

5. Cure the Patch: If using UV resin, expose it to a UV flashlight or direct sunlight until it hardens (usually 10–20 seconds). If using super glue or Aquaseal, hold the line ends together for a minute or two so it bonds. (Super glue sets in a few minutes; Aquaseal will take longer, so it’s less ideal for immediate use).

6. Let it Set: Give the repair a little extra time to fully cure before putting strain on it. With UV resin or CA glue this may be just a few minutes. Ensure the section doesn’t stick to anything while drying.

7. Go Fishing: The line should now be fishable. Keep in mind this fix is not permanent. The glued section may become stiff or crack again after repeated casting. However, it will get you through the day by keeping water out of the core and allowing you to continue fishing. Plan to do a more durable repair (or replace the line) later.

It pays to carry a small tube of UV-cure resin or glue in your kit for on-stream line repairs. UV Knot Sense is a popular choice to seal small nicks on the spot. Just be aware that any glue patch will create a slight stiff spot or “bump” that you might feel when casting through the guides.

Durable Permanent Repair

For a more robust repair that restores durability, it’s better to weld the fly-line coating using heat-shrink tubing. This approach is often used by fly line manufacturers (like RIO) to create welded loops, and it can re-form the plastic coating over the exposed core for a strong, permanent fix. Unlike a simple glue patch, a heat-welded repair replaces the missing coating and bonds it around the core.

This method works best for PVC-coated lines, which includes most floating lines like your Rio 4-weight; if the line had a polyurethane coating, a different heat-resistant sleeve technique would be used.

You’ll need some clear heat-shrink tubing—available in fly line loop kits or electrical supply stores—and a safe heat source. Here are the step-by-step instructions:

1. Gather the Materials: You’ll need a piece of clear heat-shrink tubing with an inner diameter just big enough to slide over your fly line. For a 4-weight line, tubing about 1/16”–3/32” (1.5–2mm) diameter works well. RIO Products sells “Level T” heat-shrink welding tubing for fly lines, or you can use clear electrical shrink tube of similar size. Also have a heat source—such as a 60-watt incandescent light bulb or heat gun on low—ready. Don’t use an open flame, like a lighter, because it can easily overheat and scorch the line.

2. Prep the Line: Clean the damaged section of any debris. If any loose coating remains around the gap, you can trim away frayed bits with a sharp blade. Ensure the core strands are aligned straight and not frayed.

3. Position the Tubing: Cut a length of shrink tubing about 2 inches longer than the damaged area, so it covers at least an inch past the gap on both sides. Slide this tubing onto the line and center it over the gap. The extra length will protect the surrounding line from heat and ensure a good seal past the damage.

4. Apply Gentle Heat: Slowly heat the tubing so that it shrinks down around the line and the damaged section. Hold the line about 1–2 inches above a warm light bulb (or use a heat gun at a distance), and rotate the line continuously. The tubing will contract and press the softened coating around the core. Do not overheat it. The goal is to melt the line’s PVC coating just enough that it flows and fills the gap under the tubing. Heating gradually and evenly is key. Stop heating once the tubing has fully tightened and you see the line’s coating becoming glossy under the sleeve.

5. Cool and Remove Sleeve: Take the line away from the heat and let it cool completely (several minutes) so the re-melted coating solidifies. Do not pull off the shrink tube while it’s hot. Once it’s cool, use a sharp hobby knife (X-Acto) to carefully slit and peel away the shrink tubing sleeve. The fly line’s PVC coating should now be fused across the formerly damaged gap. The core will be encased again in plastic.

6. Smooth the Repair: The “welded” area may have a slight raised line or edge where the tubing was cut. To ensure a smooth finish (so it slides through guides and doesn’t peel up), coat the repaired section with a flexible adhesive sealer. Products like Aquaseal or Pliobond work well because they dry to a flexible, rubbery finish. Apply a thin layer over and beyond the patched spot, feathering the edges. This seals any tiny gaps and tacks down the edges of the new coating. Let this coating cure fully (overnight for Aquaseal).

7. Inspect and Test: Once the adhesive topcoat is dry, run the line through your fingers or rod guides to ensure the patch is smooth. The repaired section should be waterproof and strong, with the core securely bonded to new coating material. You can now fish with confidence that the line will hold up under strain.

The repaired line might have a slight bump or stiffer feel at that spot, but it will cast and float close to normal.

Repair Kits and Products

When performing the above repairs, a few specialized products can make the job easier and ensure a reliable fix. These items are recommended by major fly-fishing gear companies and experienced anglers:

RIO Heat-Shrink Welding Tubing: RIO Products sells heat-shrink tubing specifically designed for fly-line repairs (used for making welded loops). It comes in small diameters (1/16″ to 3/32″) ideal for 4- to 8-weight lines. This tubing is used in the long-term fix to melt and reform the line’s coating.

Loon UV Knot Sense (UV Repair Resin): A UV-curing adhesive gel that’s perfect for quick fly-line fixes. It stays slightly flexible when cured and sets in seconds under UV light, making it great for stream-side repairs of small cracks. Apply a drop, expose it to sunlight or a UV lamp, and you have an instant patch.

Aquaseal® or Aquaseal FD (Urethane Adhesive): A flexible, waterproof adhesive commonly used for wader repairs, it is also recommended for fly line coating repairs. It cures more slowly (several hours) but forms a durable, rubbery seal over the damaged area. Aquaseal is excellent for smoothing and reinforcing a repaired spot in the long-term fix (Step 6). It remains flexible and won’t crack easily during casting.

Flexible CA*Glue (e.g. Loctite 414 or Zap Flex-Zap): These are cyanoacrylate superglues formulated to stay a bit flexible. A tiny drop will quickly seal a cut in the coating.

Fly Line Repair/Loop Kits: Some tackle suppliers offer kits for DIY fly line repairs. For example, RIO’s DIY Welded Loop Kit—which includes heat-shrink tubing and instructions—can serve double-duty for repairing mid-line coating breaks. These kits typically include materials and guidance to ensure a secure fix.

I hope that this helps and allows you to get back on the water those few times a year you’re casting for freshwater species.

Good Luck!

Phil