DIY Fiberglass Boat Repairs

boat booboo

Surface damage is easily repaired by DIY boaters with the right tools and know-how.

Most fiberglass boats today are made with a foam core sandwiched between layers of polyester resin. This building technique has proven strong and durable. The inevitable gouges, small scrapes, and holes your hull is likely to get are easily repaired with a little patience and do-it-yourself know how.

Take my friend Lee’s boat, for example. Last summer he scraped his 25-foot SeaSwirl on a rock while fishing. The hit was enough to scare him, peel off the bottom paint and gouge the gelcoat in one area, but otherwise didn’t appear to break through into the core material. In consulting with him on how to repair the damage below the waterline, I stressed how important it was that he make sure no water got into the core—which can result in freezing in cold climes and delamination — breaking the bond between the polyester skins and the core, greatly weakening the hull.

First, he visually checked the area for cracks. He saw none. I advised him to remove the bottom paint around the damage, rough it up with 80-grit sandpaper, and fill the gouge with WEST System Epoxy from Gougeon Bros. If he had been at all unsure of the integrity of the core, he had two options– using a moisture meter or the more direct approach of drilling a small hole to let out any water. This last technique, particularly since you are epoxying the damage anyway, will give you peace of mind.

Epoxy will stick to polyester, vinylester, or epoxy resin so if you are unsure what your boat is made of, it’s a safe bet for most repairs. I put additives (often referred to as “fillers”) in the mix to aid the repair. Each epoxy manufacturer has their own line of fillers: microfibers for stronger bonds, thickeners for upside-down surfaces like boat bottoms, and fairing additives for softer, sandable finishes.

west epoxy

I prefer epoxy over polyester fiberglass. Yes, it is more costly, but it has better strength and adhesion properties.

For Lee’s boat, I recommended that, after mixing the epoxy, he add enough colloidal silica thickener to make the consistency like peanut butter, then trowel it on to to fill the gouges in his boat’s bottom, using a piece of wax paper to keep it from sagging, and creating just enough surface tension to hold the epoxy in place until it cured. Since the repair was below the waterline, a simple coat of bottom paint would complete the repair. If the repair had been in the topsides, polyester gelcoat to match his boat’s gelcoat color would have completed the repair. Now he has no more worries, a strong, good-looking waterproof repair for under $100, plus enough supplies left over for that next inevitable fiberglass repair.

Epoxy Tips

  • Wear disposable surgical gloves.
  • Sand the repair area to create a mechanical bond.
  • Clean the area around the repair with acetone before applying epoxy.
  • Acetone will also help “wick” water out of a wet core as it vaporizes. Make sure the core is completely dry before proceeding.
  • Make small batches in disposable pint containers (10 pumps of resin is usually my biggest batch size).
  • Stir well when mixing resin with hardener, and particularly when adding fillers.
  • It’s OK to drill holes into the core in the vicinity of the repair to make sure the core is dry.
  • For applying epoxy to fill holes, use a sharp pencil to drip fill neatly.
  • Use fillers per manufacturers’ suggestions to facilitate the repair.
  • If the repair area is too large to fill in one application, you can keep adding new batches to the not fully cured (“green”) repair, but if the repair dries completely, you have to sand the surface again to remove the “amine blush” (looks like a light oil sheen) and create a new mechanical bond for future coats.
  • Temperature should be at least 60 degrees F, but heat lamps can help in questionable conditions.
  • Epoxy will break down in sunlight (UV), so apply paint or gelcoat after curing.
    Here are links to my more extensive core repair blogs.

Fiberglass Core Repairs, Part I

Rotten to the Core: Fiberglass Core Repair, Part II

I’ll address small surface gelcoat-only repairs in a future blog.


  1. Bob Fisher says:

    Its obvious that you might know how to fiberglass but suggesting using west systems (which is a great product but is epoxy) is much harder to work then using a polyester resin that much easier for the purpose you are describing. West systems epoxy is a great product for a structural resin.
    You show a picture of a boat that the glass has a hole thru the boat, at the bow. That area usually does not have a core. This area has to be to be ground and new layers of glass have to be used then gel coat to prevent moisture from going into the glass since it is hydroscopic. (Absorbs moisture). Its more work then just put in some West Systems. In my opinion That job should be done only by a professional.

  2. Brian says:


    I just inherited an aluminium hull boat, it has been painted at least once, I was wondering if you could point me in the right direction in terms of getting her back up to par?

  3. WSI says:

    Great article on making DIY fibreglass repairs. Lots of useful tips and tricks here. You may also be interested in reading this step by step tutorial on best practice for repairing fibreglass damage:

  4. Eric says:

    Has any one ever used James Hardie or pvc trim boards as core products in fiberglass reconstruction of a boat. James Hardie products are solid cement boards used as siding. I was thinking of using them as a flooring core in a 1980s Chaperal boat

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