How to use Fiberglass for Repairs – Repairing Fiberglass Cracks

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Fiberglass is a popular substance because it is light, strong and durable. Fiberglass is also flexible and can be used on things like bathtubs, roofing, airplanes, cars and of course boats. But like many things, fiberglass can be damaged by general wear and tear amongst other reasons. If you follow instructions thoroughly and take into consideration the safety precautions, fiberglass can be easy to use for repairs.



Types of Damages in Fiberglass Surfaces

Fiberglass is a mixture of thin layers of plastic, which is woven together with a type of glass wool fiber. This is also known as fiberglass composite material because it is made of two substances that come together to form one structure. Even though this material structure is durable and quite strong, it can suffer damage due to stress, age, and other factors like impact damage.

This can cause:

  • Cracks: these can be spider cracks, hairline, or stress cracks. Determining the cause is important in order to repair the damage correctly. The crack could just run along the surface or reach through the laminate.
  • Tears or splits: This happens when there is something like a boat collision or some type of impact with a stationary object. This will most likely tear through the laminate and may even damage the core structure. This type of damage is serious and would most likely require professional repairs.
  • Punctures or holes: Small holes caused by stones or other smaller objects that impact the surface. Some impact damage can be slight and only on the surface, which could easily be repaired. Other holes may penetrate deeper and cause more serious damage.
  • Damaged core: Sometimes damage can be done to the core material on impact, but the laminate layer and fiberglass surface may look intact. This can sometimes be seen as a simple cosmetic problem, an indentation, but is much more serious. The fiberglass is weak and can easily succumb to a hull breach.



Reasons for Damages to Fiberglass

When repairing fiberglass, the first thing you need to do is ensure you know the cause. This could show if the damage was caused by something outside the boat, or if something went wrong inside. The cracks can also indicate if the damage was caused by a structural defect. In many cases, poor fiberglass repairs can also result in damages becoming worse or even dangerous.

More reasons for damages to fiberglass:

  • Delamination: this is when the layers of fiberglass begin to separate due to the age of the boat, overexposure to the elements and UV light.
  • Older boats have a lot of wear and tear.
  • Weather or the elements and day to day impact forces
  • Light or UV exposure
  • Heavy loads



Materials needed for Fiberglass Repairs

First, make sure you follow safety precautions, as fiberglass can be dangerous to work with. You don’t want it getting in your eyes or breathing it in. Get yourself a respirator and overalls that cover your entire body. Also, gloves and protective eyewear are advised. If your skin starts to burn while working with fiberglass, make sure to read our article on how to get Fiberglass out of skin.

Following are materials needed for all your fiberglass repairs.


You can also purchase a fiberglass kit for repairing smaller holes and cracks.

fiberglass repairs


Fiberglass Repair Kit

The kit should contain everything you need from the fiberglass resin, hardener, fiberglass cloth as well as other items like a tool to spread the resin with and mixing container.

Other tools required:

  • Acetone
  • Paintbrush
  • Measuring cup
  • Sandpaper and sanding block


Orbital Sander

There are different types of sanders used for a variety of applications. The orbital sander is great for flat surfaces and is so named because of its orbital rotation pattern. In other words, the heads move around in small circles or orbits. They are usually tools used to remove the varnish or paint off wood and preparing a surface for painting. In many cases, a job may require different types of sanders. The orbital sander is also used in the preparation of surfaces when repairing and smoothing out a fiberglass surface. The sander can use sandpaper that has different textures from very coarse to fine grit. 40 grit sandpaper is very course and as you move up in number the finer the sandpaper becomes.



Buying Fiberglass for Repairs

Here are tips when it comes to purchasing fiberglass

  • Purchase your fiberglass in rolls. You might find that some who sell fiberglass find it cheaper to package and store folded pieces. Any fiberglass material that has been folded weakens the strands and is already damaged before you even use it.
  • Make sure you buy the hardener along with the resin. Fiberglass kits should be complete with all products needed for the job.
  • Make sure you have proper mixing containers or cups. Small batches can easily be mixed in medicine cups, which already have measurements marked along the side. You can also buy special larger cups that also have measurements on the side.
  • Make sure to purchase the correct fiberglass for a specific job. The chop mat fiberglass is cheaper, but it is not as strong as other types of fiberglass and it is not nice to work with. Resin is expensive and you do not want to use all of it on lower-grade fiberglass.




Guide for Repairing Fiberglass Cracks

As mentioned above, fiberglass can develop cracks that need to be repaired. Let us have a quick look at how this can be done.


Removing / Cutting away the Damaged Area

First, have a good look at the damage to see exactly what is wrong and then prepare the surface. You will have to remove any damaged material, try not to remove too much in the area unnecessarily. Otherwise, if it is only a small crack with little damage, drill two small holes at either end of the crack to prevent it from spreading further. In the case of a hairline crack, widen this a little with a rotary blade. This is just to make more of an area for the epoxy resin.


Cleaning the Area

Keeping the area clean, is important as you do not want any dust or particles getting in the way creating an unsuitable bonding surface. Acetone is great for this job, as it does not leave any residue behind. Wipe the area clean with a rag and a bit of acetone.


Mixing the Resin

Next, you want to mix your epoxy resin components. You will need to follow the instructions given as you want the proportions of resin to hardener correct. These products should be easy to find at your local hardware store or retailers. Marine supply companies should also have supplies. Mix your materials in a container. Only make as much mixture that you will need, as you do not want a whole bucket of epoxy you are not going to use. Fill the crack and smooth over with a plastic applicator. Allow this to dry for a day, then smooth the area over with sandpaper.


Choosing the right Fiberglass Mat

In certain repairs, you will need some type of reinforcement to the repair area. The fiberglass provides durability and strength. The fiberglass Mat is also recognized as Chopped Strand Mat and consists out of short fiber strands, which is all held together by a resin binder. This fiberglass is used in helping to build up thickness and repair surfaces. The type of fiberglass you want, all depends on what you want to be done. Do you need strength, or do you need to get around corners? The fiberglass mat is great to use around difficult corners, better than other woven materials.

The material is also ideal for the first layer before the gel coat is applied. The fiberglass mat helps prevent the weave texture from showing through and provides a smoother finish. You can purchase the Fiberglass Mat according to weight, depending on what you need it for. You also get the Fiberglass Cloth, which is a lot stronger and heavier than the Fiberglass Mat. In many cases, you can use both types of fiberglass for best results.



In many cases, such as in boat repairs, you require extra strength and support. This is where fiberglass laminating comes in handy. You take a fiberglass sheet and overlay or laminate with another sheet. You apply an epoxy mixture to your fiberglass sheet, which is then placed on the material that needs strengthening. You then continue to add layers, making sure there are no bubbles left between layers. Each subsequent layer should not cure completely, only after the final layer has been added, should it be left to fully cure.

This type of process can be used for:


fiberglass resin



To make the repaired area look good and less noticeable, you can apply a color gel enamel topcoat. The gel coat provides a nice smooth finish and also acts as a protective layer. This product also comes in a variety of colors, which can match your repair surface. Finish off by polishing the repair surface.



Guide for Repairing Fiberglass Holes

Cracks to holes in fiberglass which might require a slightly different approach when it comes to repairs. Holes in fiberglass may penetrate deeper into the surface, the repair should be fairly easy to do as long as no structural damage occurred. So, it is always important to determine the cause and extent of the damage.

To create a stable base for a cover or patch, the hole needs to be enlarged. You can use a sander on a larger area, sand it down and then taper the sanding as you move out from the hole. This allows for a larger surface area and better bonding surface for the covering. You will notice that the outside section of the hole is larger than the inside. Once you are done sanding, remove any dust with a rag and some acetone.


Covering the Hole

The next step is to fill the hole with an epoxy mix. This mix includes an epoxy resin, hardener, and filler, which should be mixed according to instructions on the packaging. The amount depends on the size of the hole that needs to be repaired.


Apply Fiberglass Mat on Top

Whenever you work with fiberglass, always remember to wear protective gear. Cut out several layers of fiberglass mat or cloth. The type of mat or cloth will depend on the application needed. Finish off the top outer layer with the fiberglass mat.


fiberglass mat



The first layer of fiberglass sheet should just fit over the smallest point of the hole. All following layers should then be a little larger, forming a tapered effect. There should be enough layers to fit the thickness of the hole being repaired. Mix your epoxy and hardener and brush the area being repaired.

Next, brush the first layer of fiberglass sheet and place it down, brush another piece of fiberglass and place this onto of the first layer. Carry on layering until you end with an epoxy layer. Gently push down after each application to remove air bubbles. To contour your repair work, you can use a backing made out of cardboard or other material before adding your layers of fiberglass.



Allow the epoxy to set or cure fully before you sand it down. Read the directions for your epoxy resin, which should tell you how long to wait. Sand the area until it is smooth. You can also add a colored gel coat, which will enhance the finished look. Finally, polish the repaired area.



Questions and Answers

Following are a few questions regarding the repair of fiberglass.


How to Repair a Fiberglass Tub

You have your traditional bathtubs made of cast iron, which is coated with porcelain, or your stainless-steel variety. Today, you will more than likely find that your tub is made from fiberglass. These may be more affordable and durable, sometimes over time, the material can crack or be damaged.

In most cases, repairing a fiberglass tub can be an easy DIY job. You can easily acquire an epoxy kit and gather a few extra materials to fix any holes or cracks.

Quickstep guide for repairing your tub:

  • Assess what the damage is if it is a simple scratch, small hole or cracks you can fix it yourself. If the damage is more serious, you might need to get a professional to determine the extent of the problem. You may have to deal with the underlying structure and require a more thorough repair job.
  • Prepare and clean the area. Always make sure to have a clean surface otherwise there will not be good adhesion.
  • Sometimes a filler is only needed for small holes
  • Drill if you need to or grind a bevel into the repair area to enlarge the surface area.
  • If it is a larger hole cut a piece of fiberglass patch material to place over the hole
  • Mix the epoxy and hardener in an appropriate cup or container
  • Spread the mixture over the crack or hole
  • Allow to fully cure
  • You may need to add a color gel to match your existing color
  • Finish and polish the surface


How to Repair Fiberglass Boat Hull from the Outside

Serious damages may require a professional in to repair your boat, but small holes or scratches may be something you can do yourself. You might be able to repair your boat hull in-water if the damage is above the waterline. Otherwise, you will have to remove your boat from the water in order to do repairs. Gather the materials required including a patch kit and you are ready to begin.

Quickstep guide for repairing your boat hull:

  • Clean your boat and the area needing repairs and remove or cut away any damaged material.
  • If the edges on the hole are on the rough side, you will have to sand down the edges. If there is a hole that extends through the hull, you will have to acquire a fiberglass backing.


repair fiberglass boat hull


  • Follow the instructions on your repair kit.
  • You will need to cut pieces of fiberglass material, which should be included in the kit. You will cut the material in layers.
  • Combine the epoxy mixture provided and apply to the prepared surface
  • Apply the epoxy mixture to fiberglass patch sheets and place over the damaged area, layer by layer. Do not let the epoxy mixture fully cure before each layer, otherwise, you will have to sand it down each time.


how to repair fiberglass boats


  • Allow curing fully
  • Sand the area for an even surface
  • Add a colored gel coat, you can usually choose the color that matches your repair surface.
  • Finally, sand, polish and wax the area


How to Repair Fiberglass Boats

Your fiberglass boat might have more damage than just to the outside of the hull. Sometimes, damage can occur to the actual core structure of the boat. This requires a lot more knowledge, time, and money. You should consider if you need a professional for this. Fiberglass boats, in general, are more durable than their wooden counterparts. But like with everything, damages and problems will occur. The first step in repairing your fiberglass boat is to assess the damage. Figure out if you can repair the damage or if something needs to be replaced completely.

Once you have identified the problem, you can remove any damaged areas. You can then prepare the area for bonding. Replace any core material with the same that was removed, keep it the same thickness and shape. If the old fiberglass skin is intact, you can use it again. Otherwise, create a new fiberglass skin over the core material in multiple tapered layers of fiberglass and epoxy.

If the only way to repair your boat is to replace damaged material, you want to make sure you do a good job. You do not want to have to come back to the problem and pay even more. The best option might be to get in a professional and only pay once.