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How to Repair Fiberglass

Fiberglass is glass-reinforced plastic (GRP). Sometimes also referred to as glass wool, it is thin fibers of plastic (polymer) woven together with glass strands. You can find fiberglass in numerous products from cars and boats to shower stalls and insulation. Sometimes fiberglass is combined with an epoxy or other bonding material to shape and strengthen it.

Safety Precautions

When working with fiberglass, always wear a face mask so you do not inhale any glass particles. It is also recommended that you wear sturdy work gloves so the glass strands do not scratch or cut your skin. Wearing long sleeves, pants, and close-toed shoes is best practice to ensure that none of the toxic resin used to patch fiberglass gets on your skin.

How to Repair Fiberglass

If you want to fix holes and cracks in fiberglass, it is possible to do it yourself. And, if you are willing to spend the time and effort doing the repairs yourself, you may end up saving a lot of money.

Preparation Tips

To make your repair job as easy as possible, ensure you have the time, tools, and materials set aside to start and finish your project.


If this is your first fiberglass repair project, it is best to set aside a day so you do not feel pressured to rush through the steps. And if you get done in just a few hours, all the better! You will also need to check your resin product to see how long it takes to set.

Tools and Materials Needed

Having the right tools and materials on hand is essential to accomplishing your repair job as smoothly and efficiently as possible. The following lists what you will most likely need to have on hand for a typical repair job:

  • Safety Glasses
  • Chemical-Resistant Work Gloves
  • Respirator (or at least a face mask)
  • Angle Grinder and Grinding Disc
  • Orbital Sander and 80 Grit Disc
  • Small Paintbrush
  • Duct Tape
  • Stir Sticks
  • Scissors (to cut Fiberglass Cloth)
  • Permanent Marker
  • Rags
  • Plastic Mixing Bucket (if it has volume measurements, it will be easier to mix in)
  • Fiberglass Cloth (or, if you need thickness more than strength – Chopped Strand Mat)
  • Fiberglass Flake
  • Bonding Resin
  • Resin Catalyst
  • Mineral Spirits (for cleaning)

How to Fix Cracks and Holes in Fiberglass

The following five steps will help you fix the cracks and holes in your fiberglass:

  1. Prepare the surfaces.
    • Find and mark all the holes and cracks in the fiberglass. Using a permanent marker, circle each damaged area so that the mark is approximately an inch outside the hole or crack.
    • Clean the marked areas with soapy water. Dry the areas with a rag.
      • If silicone or wax is around the marked areas, you may need to use a residue remover.
    • Grind the damaged areas down. When the areas are dry, use an angle grinder with a quality grinding wheel to grind down the surface within each marked circle. The idea is to taper the surface from the marked circle down to the hole or crack, so that the edge of the hole or crack is a thin sharp edge.
  2. Prepare the patches.
    • Cut patches out of the fiberglass cloth for each hole or crack. The patches should be cut twice as big as the damaged area. It is easier to then scale down the piece to your desired size, if necessary.
      • If you have any small drill-size holes, you can repair those later using chopped fiberglass flake and the remaining resin. You will simply need one patch that is a little larger than the hole. (See Step 5.)
    • Determine how many layers of patches you need. Check how thick the fiberglass shell is around the damaged areas. If you need multiple patches to match the thickness of the fiberglass shell, cut multiple pieces out of the cloth so you can stack them. At least one of the pieces should be roughly the size of your marked circle. Then, the other patches can step down in size. However, the smallest size of patch should still be bigger than the hole or crack it is covering. (The patches do not have to be perfectly cut out.)
  3. Mix the resin.
    • Determine how much catalyst to add to the resin. This step requires thorough attention to detail. Carefully follow the instructions on your container of resin. Typically, resin curing works best in stable environments of approximately 70 degrees Fahrenheit with good airflow.
      • If your project is located in a colder environment you may need to use more catalyst than is recommended. However, if you use too much catalyst, your resin may become brittle and crack.
    • Add the catalyst to the resin. Remember to stir the resin while you add the catalyst and to continue stirring the mixture for one minute after. As you stir, scrape the bottom and the walls of the container to ensure the catalyst mixes into the resin well.
  4. Glass in the patches.
    • Use a small paintbrush to apply resin to the damaged area.
    • Add a layer of resin to the largest patch and apply the patch (resin to resin) to the damaged area of fiberglass.
    • Continue applying resin to each subsequently smaller patch and place them on to the previously applied patch until you have added all the patches to the damaged area.
    • Clean up any resin drips before they harden. Use a rag dipped in mineral spirits to wipe up the resin drips.
  5. Fill small holes.
    • Back any small drill-size holes with duct tape.
    • Mix some chopped fiberglass flake into the remaining catalyzed resin.
    • Fill in the hole, using the paintbrush dipped in the flake-resin mixture.
    • Seal the hole with the small patch.

Tips for Fixing Fiberglass on Boats and Cars

While repairing the fiberglass on boats and cars is similar to general fiberglass repairs, it is important to consider the following:

  • For small fiberglass repairs, people often use polyester resin. However, the porosity of polyester resin may not be suitable for items that could frequently encounter water like boats and cars. So, choose your resin carefully.
  • When repairing extensive areas of damage on boats or cars, it is recommended that you use fiberglass cloth rather than fiberglass filler.

Common Errors

Before you begin your fiberglass fix, read through the following common errors people make to ensure you do not make the same mistakes:

  • Not matching the thickness—When you are repairing a damaged area of fiberglass, you need to ensure the patch/fill thickness matches the original. If you fail to do this, the area around the repair may crack.
  • Missing the root cause of the damage—If the damage was caused by an outside source (e.g., a rock hitting a boat), then simply repairing the damaged area should be sufficient. However, if the damage was caused by an “internal” issue (e.g., something else wrong with the boat), then you will first have to address that issue before you can successfully patch the damaged fiberglass. Otherwise, the repair will only be temporary.
  • Not accounting for backside or inner damage—Surface damage can appear minor, but there may be a greater impact on the backside or inside of the fiberglass. Be sure to thoroughly check the damaged area so you can thoroughly repair it.
  • Creating sharp corners—It is best to create rounded shapes, because sharp corners can be stress concentrations and weaken the repair.
  • Using excessive heat when grinding—While it is important to prepare the area by grinding it down, excessive heat can soften the surface. This can make it difficult for the patch, resin, etc., to adhere to the surface.
  • Not removing excess damaged material—The repair patch must be adhered to undamaged laminate for it to properly bond. Remove all damaged laminate before you proceed with the repair.
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