how to repair fiberglass cracks

How to Repair Fiberglass Cracks Quickly and Easily

Also known as glass reinforced plastic, we use fiberglass to build everything from airplanes to boats. Those working with the material love it for its unique composition that combines strength with flexibility.

But like anything else, fiberglass is prone to breaking under insentience conditions or long periods of wear. The material can and will crack depending on the circumstances its faced with.

Though that doesn’t mean that cracked fiberglass is worthless. Far from it, actually. To help ensure your fiberglass lasts for years to come, we’re breaking down the best ways how to repair fiberglass cracks quickly and easily.

What is Fiberglass?

Most people know fiberglass as the stuff in your walls that splinters off into your fingers. And while that’s fiberglass insulation, it’s not what we’re talking about today. Instead, we referring to thin fibers of woven polymer plastic.

As we said above, fiberglass is reinforced plastic. Polymer (plastic) fibers are woven together with strands of glass to make the end product, fiberglass. It’s then shared with a mixed material like epoxy or thermoplastic to add shape and strength.

The end result is the material see that helps us create boats, planes, showers, insulation, and more. The various states of fiberglass are thanks to the different mixtures that result in different densities.

Fiberglass Safety

It’s important to note that despite fiberglass’ many uses and overall ability to retain its shape and strength, it does come with some safety hazards. Depending on the mixing material, fiberglass is prone to releasing glass particles.

Harder mixtures seal these particles together, but insulation and even cracked pieces of harder mixtures can release glass into the air. Always wear a mask when working with fiberglass.

Likewise, glass particles are also prone to flake off of broken fiberglass. The glass strands easily dig into any exposed skin, making gloves another must for working with fiberglass.

How to Repair Fiberglass Cracks 101

Repairing fiberglass cracks has much to do with what you’re repairing and the size of the crack in question. For today’s guide, we’re keeping it to something you’d see in your house, while also throwing in a few tips for bigger repairs (like cars or boats).

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Getting Started

It’s best to start with all the materials you’ll need to fix the crack. Always start over prepared. The basic repair kit includes:

  • Polyester (Fiberglass) resin with accompanying liquid hardener
  • 80-grit sandpaper
  • Gloves
  • Mask
  • Mixing container
  • Colorant (for bathtubs etc.)
  • Finish (for bathtubs etc.)
  • Fiberglass Mesh (for cracks over a quarter-inch in width)
  • A Drill

While it might seem like you can forgo colorants and mesh, it’s better to err on the side of caution. The resin cure is time sensitive, meaning you can’t just run back to the store in the middle of your patch.

The Repair

First, you need to clean the crack. Rubbing alcohol works well to strip away and dirt, grime, or leftover oils and such from the crack.

Next, drill out two holes, one on each end of the crack. This stops the crack from spreading further. If your crack is over a quarter-inch wide you’ll also drill holes on either side of the crack to provide anchor points for your mesh.

You can fill the holes with foam to create a backing if you’re working with fiberglass that isn’t set onto anything. Think the hollow part of your tub. This is also important

Now it’s time to measure out your piece of fiberglass mesh (if you’re working with a larger crack). Cut the mesh to cover the drilled holes as well as the crack. Next, we’re mixing resin.

Start by adding the hardener and resin mix in measured parts according to the instructions provided with your products. Though do note that the proper ratio is based on a temperature of 76 degrees Fahrenheit.

Mix only what you can use in 15 minutes as the resin cures beyond use after that time frame due to the exothermic nature of the curing process. In laymen terms, curing heats up the mixture which makes it cure progressively faster.

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Spread the cure through the crack and over the drilled holes. Next, place the mesh over the holes and crack (behind the front surface). If you’re not using mesh just fill the holes and crack with the resin mixture and wait.

If you are using mesh, apply the mesh over the first coat of resin and then paint more resin over the mesh. During the curing process, the mesh should become invisible. Don’t hesitate to use excess resin to really fill the cracks.

Now it’s time to wait out the recommended curing time listed on the resin mixture’s package. The longer the curing the harder the end product, to an extent.


Finishing your repair job varies depending on what you’re repairing. Projects that are always visible take more finishing care than those hidden once you’re through. For instance, a fiberglass shower will take more finishing than a car bumper.

First, sand down the excess resin cure off the former crack. If you’re fixing something visible, feel free to use any finishing paint or stain of your choice to further mask your repair.

After that, you’re all finished. Clean your workspace and check on the integrity of your repair. If you’ve followed our guide you should find yourself with one less cracked piece of fiberglass. And if you have more question you can always learn more.

Advanced Repairs

Sometimes repairing fiberglass goes beyond the skills of the average person. That’s fine and even common. Large and complex cracks often compromise the entire structural integrity of the object in question.

In these complex cases, there’s no shame, and it’s even wise, to contact a professional.

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