There is a crack about 12 inches (30cm) long on the edge of my Sunfish sailboat. I am not sure if the crack is all the way through the fiberglass, or just the gel coat. It is on the corner between the side of and the bottom of the boat. When I push on it I can hear a little noise that could be glass fibers moving around. I suspect the fiberglass is at least partially compromised.

Googling around I find people who are using lots of products on fiberglass with super glue and gorilla glue, often considered but maybe not the best choice.

Pending a proper fiberglass patch with a new gel coat application, is there anything I can use to fill and join the crack, that will work well and not create a problem later?

I am looking for a solution between ducktape and a full fiberglass patch as described in these two questions How to fix a leak in a fibreglass kayak? & I put a big gouge in the bow of my canoe going across rocks. How can I fix it?

Image of Sunfish, upside down showing crack

enter image description here

  • A gel kit is like $30. Why are you opposed to a kit?
    – paparazzo
    Commented Oct 23, 2016 at 16:50
  • @Paparazzi I assumed a gel kit would only patch cracks in the gel coat and is not appropriate if the crack continues into the fiberglass. Am I mistaken? Commented Oct 23, 2016 at 20:27
  • 1
    On fiberglass if its not a field repair to get you home do a proper patch from the start: anything else can make the application of a patch more difficult as you might have to eliminate completely what was put on (various other glues etc) to get a proper structural bond. Commented Nov 3, 2016 at 12:41

3 Answers 3


Gelcoat cracking is pretty much a fact of life in boat ownership. The sun and water is a tough environment. However, gelcoat cracking is usually not too concerning as long as it is fixed right away.

While you could probably get by for awhile just slathering some new gelcoat or epoxy over the crack, if there is any moisture trapped in the cracked area, you're going to make the problem worse. If you live in an area with a real winter and water freezes in the hull you're going to make it a lot worse.

The short answer is to get one of those little fiberglass repair kits from your favorite boating retailer, and follow the directions in it. The only thing that will work well is a proper fiberglass repair, that's why they sell the kits. The basic process is to remove and sand away any compromised material, and build it back up with new material provided by the kit. In addition to the kit's directions, there are roughly a gazillion guides online about the subject.

Interestingly enough, the kits usually don't include gelcoat, so you'll need to get some of that too. Epoxy's UV resistance is pretty lousy and unless you protect it with gelcoat or some other product you'll be back at square one in a few years.

The kits usually use epoxy, which 15 years ago could have been a problem, as sunfishes (and many other boats) are made with polyester resin instead of epoxy, which doesn't bind together too well, but epoxies nowadays have supposedly made that problem a thing of the past.

The fiberglass repair isn't terribly difficult, and unless you are trying to sell the boat and never sail again, I guarantee this won't be the last repair you ever make, so you might as well learn how it's done.

  • +boat always = +$. Can I use the automotive fiberglass repair kit? Commented Oct 24, 2016 at 14:01
  • Actually it looks like bondo resin from the auto parts store might be a better choice bondo.com/bondo-fiberglass-resin-401.html#6MCtHTHxlrbvISII.97 Bondo® Fiberglass Resin is the same high-strength polyester resin that is used to build most boats. Commented Oct 24, 2016 at 16:48
  • @JamesJenkins: is there a particular reason you are resisting going to a store that has the right supplies? Commented Oct 24, 2016 at 23:05
  • I went to a marina Saturday morning, to look for a drain plug and polyester line. They had three drain plug choices, and only nylon rope, they suggested a different store north of the city. I drove there, waited 40 minutes for them to open, and found a drain plug candidate, they had "poly" neither the salesman nor the tags defined if it was polypropelene or polyester. The line was twice as expensive, as the known right stuff online. I left with the drain plug which didn't work. Today Monday, I drove 45 minutes to the the store to return the plug, it was closed, the right store is not near me Commented Oct 24, 2016 at 23:53
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    As for the polyester, the problem with polyester vs epoxy wasn't that epoxy would fail to bond to polyester, but the other way around, with subsequent polyester failing to bond to epoxy in a later repair, and so once someone used epoxy there was no going back. Commented Oct 26, 2016 at 21:14

The gel coat is cracked. It is appropriate to patch the gel coat. As long as the fiber glass stays wet it is just going to get softer. Take down the gel coat enough to expose some fiber glass and get a smooth edge. Get the fiber glass dried out and then seal (with gel coat).

  • I think you meant this to be a reply to James Jenkins comment. Commented Oct 24, 2016 at 1:34

The right way to do it is to repair it as suggested with a fiberglass kit. However, as also mentioned, you'll need to put gel coat on to seal the fiberglass. If you go this route, I suggest buying one of Don Casey's books on hull repair.

Another option that I've used is Marine-Tex. I stress fractured my foredeck by stupidly cranking down the forestay too tight. I ground out the crack all the way to the wooden core and then filled the grind area with white Marine-Tex being careful to leave no voids. The repair isn't perfect, but unless you're looking for it, you're unlikely to see it. I've also repaired minor dings in the deck with it, and filled holes in the cabin left by the previous owner's sloppy VHF installation.

  • +1 but, Looking at marinetex.com/marinetexepoxyputty.html seems like for the cost and effort, you might as well do the fiberglass repair and cover it with gelcoat. Commented Oct 24, 2016 at 14:22
  • I'd largely agree, Marine-Tex is a bit of a cheat, it gets the job done, and works well, but as I said, the right way to do it is the fiberglass repair. I happened to have a Marine-Tex kit on the boat when it happened, so instead of making a trip back home (and back, roughly 90 minutes) for fiberglass, epoxy, and a trip to a store for gelcoat, I took the easy way out.
    – delliottg
    Commented Oct 24, 2016 at 16:26

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