How to Repair and Patch Fiberglass on Your Boat Like a Pro

One of the worst feelings on the water is discovering a crack in your hull, a blister on its bottom, or a gouge in the topsides. Whether you’re miles offshore or just pulling off the trailer, fiberglass damage stops fun in its tracks. But here’s the good news: most fiberglass repairs are totally DIY-friendly with a little know-how and the right gear.

At Sea Tow^®, we believe knowledge is power, and confidence on the water starts with being prepared. This comprehensive guide breaks down everything you need to know to tackle fiberglass repairs yourself; from identifying damage to applying patches and restoring your gelcoat shine. Whether you’re patching a dock ding or fixing a full-on breach, this one’s for you.

Types of Fiberglass Damage

Not all damages are created equal. Before you pull out the resin, it’s critical to understand what you’re dealing with. Here are the usual suspects:

• Hairline Cracks: Common in high-stress areas or from minor impacts. Often cosmetic but can become worse if ignored.
• Spider Cracks in Gelcoat: Looks like webs, often from flexing or age. Usually surface-level but can let in water over time.
• Blisters: Those little pimples under the waterline? That’s osmotic blistering caused by water permeating the gelcoat.
• Delamination: When layers of fiberglass separate. You’ll sometimes hear a dull “thunk” on a tap test or feel a spongy spot.
• Holes & Punctures: From run-ins with docks, rocks, or careless tools. Ranges from cosmetic to structural.
• Gouges & Abrasions: Dragging against something hard? These can rip away protective layers and expose core materials.
• Warping: Usually from prolonged sun exposure or heat. Watch out for distorted panels.
• Fatigue & Flex Cracks: Boats that take a pounding offshore often show signs of wear in the form of flex damage.
• Stringer Damage: Rot, delamination, or impact here can compromise the boat’s core strength.
• Gelcoat Wear: Chalky finish? Fading color? UV radiation can take a toll.

💡 Pro Tip: Use a coin to tap around the area. A hollow or dull sound could indicate hidden delamination.

Tools and Materials for Boat Fiberglass Repair

Fiberglass work isn’t rocket science, but it’s precise, and having the right materials is essential. Here are the necessities for your toolkit:

Fiberglass & Resin:

• Fiberglass Cloth/Mat: Provides structural reinforcement to the damaged area; different types suit different resins and repair needs. Choose woven cloth for strength and mat for buildup. Layer them for larger patches.
• Resin (Epoxy, Polyester, or Vinylester): Acts as the bonding agent, holding the fiberglass reinforcement together and adhering it to the existing boat structure; different types offer varying strengths and compatibility. Epoxy is strongest and best for structural repairs. Polyester is more common and cost-effective.
• Hardener/Catalyst: Initiates the chemical reaction that cures the resin. It’s specific to the type of resin used, so be sure to use a match.
• Fillers: Thicken resin for filling gaps, creating structural fillets, or fairing surfaces to a smooth contour.
• Gelcoat Repair Kit: Contains materials for repairing damage to the outer gelcoat layer, including colored paste, catalyst, and color-matching pigments.

Prep & Application:

• Acetone & Tack Cloth: Remove dirt, grease, wax, and other contaminants from the repair area to ensure proper adhesion. Resin won’t bond to grime.
• Sandpaper (80–600+ Grit): Used for preparing the surface, shaping the repair, and achieving a smooth finish.
• Brushes, Rollers, and Spreaders: Apply resin evenly.
• Syringes & Tape: For injecting resin into small cracks or delaminated areas.
• Release Film (Mylar or Wax Paper): Keeps gelcoat smooth while curing.

Finishing:

• Orbital Sander & Sanding Block: For more efficient sanding of larger repair areas.
• Polishing Compound & Rubbing Compound: Used after sanding to restore the gloss and shine to the repaired gelcoat surface.
• Gelcoat Wax/Sealant 

💡 Bonus Tools: A heat gun can speed up curing. A moisture meter can confirm water intrusion. A coin tap tester helps find delaminated spots. Keep a HEPA vacuum handy to clean up dust safely.

Safety Precautions You Can’t Skip

Working with fiberglass and resin means you’re dealing with tiny glass fibers and potentially nasty vapors. Skip the safety, and you might end up itching, coughing, or worse.

• Ventilation matters. Work outside or in a well-ventilated area with fans.
• Gloves and coveralls protect your skin from glass shards and resin burns.
• Respirators are a must when sanding or mixing resin.
• Clean up properly. Use a HEPA vacuum for dust and wipe everything down after.
• Dispose of waste safely. Bag up resin-soaked materials and follow local disposal guidelines.
• Accidents happen. Know basic first aid for resin splashes or fiberglass irritation.

How to Repair and Patch Fiberglass

Assess the Damage

Start by outlining the affected area with a marker and use the coin tap method to check for delamination. Look for soft spots or water intrusion, moisture and/or discoloration are red flags. For major structural issues, skip the DIY and call in a pro.

Prep the Surface

• Clean the area with soap and water, then acetone.
• Grind or cut away any loose or damaged fiberglass using a Dremel, grinder, or knife.
• Bevel the edges for structural patches, sand a taper (12:1 ratio) around the damage. This creates more surface area for bonding.
• Rough it up. Sand the area with 80–180 grit to help resin adhere.

💡 Don’t skip the bevel. It’s the difference between a solid repair and a future failure.

Cut & Fit the Fiberglass Patch

Cut multiple layers of fiberglass cloth in decreasing sizes. The biggest patch should overlap the damage by at least 2 inches. For holes, back them with foam or wood temporarily while you patch from the outside.

Mix and Apply Resin

• Mix resin and hardener according to the manufacturer’s instructions. Small batches are better to avoid premature curing.
• Wet-out the cloth with resin using the dry method: lay dry cloth on the surface and then apply resin.
• Layer patches from largest to smallest. Each should be fully saturated and transparent with no air bubbles.
• Squeegee excess resin outward with a plastic spreader.

💡 Vertical repair? Use thickened resin or spray adhesive to hold patches in place.

Let It Cure

Let the resin fully cure. This might take hours or even overnight depending on humidity and temperature. Don’t rush it. Sanding too early can gum up the repair.

Fair and Sand the Surface

Once cured:

• Sand the repair smooth using progressively finer grits (start with 80, finish with 600+).
• If the surface is uneven, apply fairing compound and sand again.

Use a longboard sander or sanding block to keep the surface level and fair. Wet sanding helps get a glassy finish with less dust.

Repair the Gelcoat (If needed)

• Clean the area with acetone.
• Mix gelcoat paste and pigment to match your boat’s color. This can be tricky so test a few small batches in less obvious spots first.
• Apply gelcoat with a putty knife, slightly overfilling.
• Cover with release film and let it cure.
• Sand and polish using ultra-fine grit (up to 2000) followed by rubbing compound, then polish.

Finish with a protective wax. Done right, your repair will blend seamlessly with the original finish.

Pro Tips for Stronger, Longer-Lasting Repairs

• Test your resin mix on cardboard first to get a feel for working time.
• Dry everything thoroughly, even a little moisture can ruin adhesion.
• Warm up the area if working in cold weather (use heat lamps or fans).
• Create temporary molds for complex shapes or curves.
• Label everything. Keeping notes on materials used helps if future repairs are needed.
• Inspect regularly. Small cracks can turn into big headaches if ignored.

When to Call in a Pro

Some repairs are better left for the experts:

• Hull breaches
• Delaminated stringers or bulkheads
• Widespread moisture intrusion
• Areas under high structural loads

Attempting a more complex repair on your own can compromise your boat’s safety and long-term performance. That’s why sometimes calling a professional isn’t just a smart choice, it’s essential.

Sea Tow Team

Sea Tow has been the premier leader in on-water boating assistance since 1983. We want to share news, press, tips and all things boating.