How to Repair Fiberglass Cracks on Boats

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How to Repair Fiberglass Cracks on Boats

One of the common problems with fiberglass boats is that fiberglass has the tendency to crack frequently. Most fiberglass cracks on boats are caused by stress. Stress cracks are usually attributed to the gel coat that lies below the fiberglass. If there is a void or if the gel coat is not sturdy a stress crack can occur. Usually weakness in the gel coat can occur during a previous repair or during the manufacturing process. When in water your boat is subjected to constant and substantial forces, it will cause the fiberglass to flex and ultimately crack when it reaches its limits. Because the gel coat does not flex synchronously with fiberglass these stress cracks can occur. Stress cracks look like hairline cracks. They can be short in length of only an inch or two or reach across the length of the boat. A small stress crack can at anytime grow in length creating headaches for boat owners. If a stress crack is noticed, you will probably want to repair the damage before the crack lengthens.

Repairing Fiberglass with Simple Stress Cracks on Boats

The good news is that fiberglass hulls with stress cracks can be fixed. In fact, one of the best qualities of fiberglass is that it is fairly easy to repair. Stress cracks can be fixed satisfactorily by grounding out the crack. Grounding out a crack is when you use a sander or buffer to remove the outer most fiberglasses until good, uncracked fiberglass becomes present. In order to ground out a stress crack, you should buy some 36 grit sand paper and attach it to a drill or sander.

Repairing a small stress crack is quite easy to accomplish. Simply ground out the stress cracked fiberglass removing any whited out fiberglass or fiberglass that is shattered. When grounding out make sure that you not only ground out the stress crack itself, but the area surrounding it. For a small crack that is only a quarter inch thick, grind around the width of a couple of inches.

Repairing Fiberglass with Voids

A void is where the fiberglass on the boat has a dip or low spot in it. Usually in voids, the gel coat doesn’t touch the fiberglass, so it doesn’t bond with it. Like a small crack, you will need to ground out the crack. Use sand paper (36 grit), a sander or drill attachment. Ground out the area of the void all the way down until you hit the end of the void. Once the void is grounded out, you will need to apply new fiberglass that will require you to shape, mold, mask, and reapply the gel coating, etc.

How to Apply New Fiberglass

Make sure you repair your cracked fiberglass thoroughly. Any area of the fiberglass boat that was not repaired competently may crack at a later time requiring you to redo the process.

Fiberglass Crack Repair On Boats Image Gallery



  1. Abe Asuncion said,

    on July 1st, 2010 at 11:21 pm

    Good afternoon!

    We are seriously considering to replace our wooden fishing boats with fiberglass ones. Could you please advise us on the pros and cons of both before we decide? thanks a lot!


  2. Matthew fogden said,

    on April 28th, 2009 at 12:53 am

    sorry to bother you,
    we have a crack on the transom that hold the engine bracket on, would it be smart for strengthening reasons to place alloy or s/steel plates either side, or cover the hole back of the boat.