How to Repair Boat Fiberglass

Jan 3, 2022

Did you know you can repair holes in the hull or deck of your fiberglass boat? To make these strong and to near cosmetic perfection, visit a shop that specializes in boat fiberglass and gelcoat repair. A DIY enthusiast can also achieve good results with fiberglass and gelcoat repair. Learn the techniques and materials that are used to get the best results.

white boat bottom fiberglass

A hole in the side of this Boat was permanently repaired to cosmetic perfection by choosing the right fiberglass materials and applying the proper repair techniques.

What is Fiberglass and What Gives Fiberglass Strength?

Fiberglass is from two components: resin and fiber. It is called fiberglass because the structural fiber used is actually woven with tiny flexible strands of glass. The fiber is then saturated with resin, stretched into place, and when it cures through a chemical reaction, the result is a hard and durable material. The strength of the fiberglass largely depends on the weight of the structural fabric that is used. There are different weave patterns to the fabric that render it useful for different applications. The resin component is either polyester resin or epoxy resin. Epoxy resin is stronger. If the repair area is large and is in a high-stress part of a boat below the waterline, epoxy resin is the superior choice. This is because it has excellent bonding properties to an existing surface and is strong.  Polyester resin is commonly used. However, one drawback is that it shrinks as it cures and thereby diminishes its adhesive qualities. This shrinkage is insignificant in small repairs, even in high-stress areas. But because of the shrinkage and reduced adhesion, there are limitations to using polyester resin for large repairs in high-stress areas. However, outside of large structural repairs, polyester resin is the material of choice and is used in the bulk of fiberglass repairs.

fiberglass repair

For small fiberglass repairs, polyester resin is used in combination with mat or chopped cloth mixed in resin to make a sturdy repair.

Fiber Choices For Fiberglass Repair

The most common type of fabric is plain weave cloth. It is easily recognized as it is woven at 90 degrees, and it looks like cloth. It comes in different weights, and a thicker cloth is stronger. One limitation of plain weave cloth is that it doesn’t conform to complex shapes or curves. It’s best applied on an even plane. For complex shapes and compound curves that require strength, the best fabric is modified twill. Its weave appears to be diagonal, but in reality, it is also woven at 90 degrees and is a looser weave with flexibility. It can conform to complex shapes without wrinkling. The most flexible fabric is called a mat. It has hair-like strands going in every direction. Mat is bonded with glue that dissolves in polyester resin, enabling it to conform easily to complex shapes or compound curves. However, it has less strength and is often used in cosmetic applications unless built up in many layers. Lastly is a fabric called roving. This is a fabric that has course bundles of strands in its weave. It is a very heavy fabric for the strongest of applications. It will be used in combination with layers of mat to give a smooth final surface.

repairing hole in jestski

Structural fiberglass cloth and cosmetic mat are used in combination with resin to make a strong fiberglass repair.

Repairing a Hole With Fiberglass 

When repairing a hole with fiberglass, repairs are best made by access from both the inside and the outside of the repair. However, access from the interior may not always be possible because a floor or interior wall may block access.  If it’s a serious structural repair in a high-stress area, you’ll have little choice but to cut a hole through the floor or wall to access the inside of the repair. However, doing a repair from the outside only is possible for small repairs or repairs in low-stress areas.  The first thing to do is assess the damage carefully by cleaning the exterior with acetone. This will make hairline cracks evident, and you’ll want to grind all the way back until the crack terminates. First, using a 4-inch grinder with 80-grit sandpaper, grind or cut away any broken material in the perimeter of the hole that lacks structural integrity. Then with the grinder, taper the edges of the hole using a 12:1 rule. That is, taper back the edge of the hole at an angle that is 12 times the overall thickness of the fiberglass material. 

Checkout marine product sales online for all your boat repair equipment.

repairing boat hole using grinder

To repair a hole, use a 4-inch grinder and coarse sandpaper to grind away any broken material that lacks integrity. Then taper back the edges of the hole.

If it’s a repair from both the inside and outside, the taper will be centered on the thickness of the fiberglass and taper back on both inside and outside, resembling the cutting edge of an ax.  If the repair is done from the exterior only, then the taper will start at the bottom of the thickness of fiberglass and taper toward the outside, resembling the cutting edge of a carpenter’s chisel. Next, after the taper from the edge of the hole outward is complete, grind back toward the perimeter to where the hairline cracks ended. Now the entire repair area has been defined. Repeat the same on the inside of the repair if you’re doing a two-sided repair. Clean all surfaces with acetone to be free of dust so the resin will adhere.

For a two-sided repair, next, you’ll cut a piece of fabric that extends to the outer margin of the repair. Mix up resin and hardener, and on a scrap piece of cardboard, lay your piece of fabric, and use a brush to saturate the cloth with resin. Turn the fabric over and coat the other side.  Next, take the brush and apply resin to the interior repair area. Then carefully stretch the resin-soaked fabric over the hole, so it’s on a smooth plane and wrinkle-free. Use the brush to press out any air pockets, so the patch sits flat against the existing interior surface. Once the interior fiberglass patch has been set up and is somewhat hard, have already cut a piece of cloth or several layered pieces of cloth that will fill the thickness of fiberglass to near the surface plane of the exterior. The outermost layers will be a resin-soaked mat, so it will be smooth and not show the texture of the cloth. 

Let the patch dry. Once it is dry, grind down the high spots with 80-grit sandpaper until it is on the plane with the finished surface. At this point, clean off the sanded area, and assess the low spots. If there are still high spots, continue to grind them down. Next, you’ll use a resin that’s mixed with chopped strands of fiberglass. This is a cosmetic layer, as the structural layers are already in place. This will fill the low spots and bring the repair up to plane with the exterior surface. Once the chopped strand layer is set up and hard, grind down the high spots until in plane with the finished surface. The next step is to mix up what is referred to as “peanut butter .”This is resin and thickener and is the consistency of smooth peanut butter.  Using a plastic applicator, apply the peanut butter resin mixture to give the final surface all the way to the perimeter of the repair. If it is a curved surface, use a metal, plastic, or plywood flex board to smooth the final contour. Once this layer is dry, grind it down so you’re perfectly on the plane with the exterior surface. Now there should only be some minor pits and pinholes to fill. Use a skim coat resin and hardener and apply using a plastic applicator. Once dry, sand to perfection using fine sandpaper.

Spray the Gelcoat

The Gelcoat is the shiny hard finish surface on the boat exterior. Once the color is matched, spray on the finish coats of gelcoat. Once the topcoat dries, use 600-grit wet sandpaper and higher to seamlessly blend the new gelcoat with the original finish. Lastly, use wax to give the final buff to make the repair invisible.

covering the boat repairing minor chip

Gelcoat is the shiny cosmetic finish to Fiberglass repair. It is sprayed on and built up in multiple applications to create a glossy topcoat.

Repairing a Minor Chip or Gouge in the Gelcoat

In this case, the structural fiberglass has no damage, and it’s just the finish layer of gelcoat that needs repair.  Grind the repair with a Dremel tool to expose clean fiberglass and bevel back the repair edge. Mix up the correct color of gelcoat with hardener, and take a small brush and create a slightly raised puddle of gelcoat that entirely fills the void and extends just past the margins of the repair area.  Let the puddle dry hard, then using 600-grit wet sandpaper, sand until flush with the finished surface. On a flat surface, use a sanding block to create a perfect plane. End with a coat of wax to make the repair blend in perfectly.


Repairing fiberglass is a multi-step process. However, even the most unsightly damage from hitting a rock, bumping into a dock, or even a collision can be repaired to look just like new.  Lakeside Marine offers professional boat fiberglass repair from minor dings to major structural repairs. We also handle fiberglass repairs for boat insurance claims, making the repair and insurance reimbursement process seamless. We even do boat modifications and restorations. 

four professional fiberglass repairing the boat

Lakeside Marine does fiberglass repairs for boat accident insurance claims, classic boat restorations, and custom high-performance boat modifications.

For professional results for your boat fiberglass repairs, call email or go to our webpage and learn about our [fiberglass repair services], and read customer testimonials about how Lakeside Marine performed outstanding fiberglass repair while providing exemplary customer service.

Book a Boat Fiberglass and Gelcoat Repair


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