Introduction: How to Fiberglass: Fixing a Cracked Kiteboard

About: I've worked for Instructables off and on since 2006 building and documenting just about everything I enjoy doing. I am now the Creative Programs founder and manager for Autodesk and just finished building out…

This Instructable explains how to work with fiberglass cloth and resin. In this project, fiberglass cloth and resin are being used to fix a crack in a kite board, but the information included here applies to virtually anyone working with fiberglass.

I had help from my friend Danny, an experienced boat builder, but for the most part, I was working with fiberglass for the first time. Writing guides from the beginners point of view has yielded good results in past Instructables, so I am doing this one in the same style. As a result, there might be information in here that will seem obvious to the experienced fiberglasser, but that same information might be valuable to someone who is just starting out.

Step 1: Background

In its most simple form, the process of fiberglassing starts with very small strands of glass that are either woven together like a cloth, or chopped and then pressed together to form a mat. This material, which comes off of a roll, is then saturated with a two part epoxy or polyester resin that gets painted on in a thick viscus liquid. When the resin dries, the entire structure hardens and becomes rigid. Fiberglass can be used to make volumes and structures ranging from speakers to snowboards and is a valuable medium to work in when you want to create something that can't easily be cut out of wood or metal - curved 3D objects for example.

If the fiberglass is applied on a flat surface, it hardens into a flat sheet when dry and will act like any other rigid material.

However, if the fiberglass is applied over a form, say the ribs of a boat hull, then it can be used to create strong, lightweight volumes that can take on virtually any curve, contour or shape.

Because it can be used to create rigid curved surfaces, it's a process that is generally reserved for specialized building projects. See the following steps of some specific Instructables to see what I mean: step 11: Create subwoofer retrorockets and midrange 'eyes', step 16Fiberglassing your beaver, Creating a fiberglass snowmobile trailer and trebuchet03's amazing video/Instructable on making a fairing for a human powered vehicle. When you need to make something that is rigid and curved, fiberglass is definitely the way to go.


Working with fiberglass comes with a whole new set of terms and lingo that it might be useful to know. I can't say that I will be using all of these words in my Instructable, but they are good to know anyway.

Fiberglass - very thin glass fibers that are treated like threads in a textile
Woven vs. Chopped Mat - woven mats contain fibers that are aligned and woven together like fabric, while chopped mats are made up of randomly aligned short fibers held together using a binder.
Resin - resin is the syrup like liquid that wets out the mat and then dries hard. It's activated by a hardener or second part.
Polyester vs. Epoxy Resins - polyester resins are cheaper, heavier and older then epoxy based resins. Polyester resins are activated by adding around 1.5% - 2% of total volume of resin that you are mixing of a hardener (MEKP). Epoxy resins come in a two parts that are mixed in more equal ratios )generally between 1:1 and 1:4). Epoxy based resins are stronger, have better adhesive properties then polyester resins and are more resistant to cracks and water. The major drawback is their price.
MEKP - Methyl Ethyl Keytone Peroxide - polyester resin catalyst (hardener)
PVA - Polyvinyl Alcohol (film mold release)
Cabosil - the white silica filler being mixed in (trade name - aka Aerosil or fairing filler)
Tool - negative mold used to make parts
Micro Balloons - air filler encapsulated in silica glass (creates a lattice matrix structure in resin)
Gelcoat - provides a high quality finish to composite tool surface
Composite - material made from two or more substances (in this case fiber glass and resin)
Exotherm - reference to the heat generated/required by the resin to cure
Pot - Catalyzed batch of resin
Pot Life - Working/usable life of pot. After this time, the chemical reactions between catalyst and resin "take off"

**Much of this list is copied from Trebuchet03's fiberglassing glossary which originally appears here.**

Safety Gear:

Working with fiberglass is actually one of the more hazardous processes out there. The small fibers from the mat can cause skin irritations and tend to get all over the place. Fumes from the resin and cleaners that are used can cause long term damage and are full of all kinds of nasty VOC's (things that mess up the brain and the baby maker). And then there is the large amount of very fine dust that is created when sanding. It's not that working with this stuff is deadly, it's just that it must be dealt with in the proper way and using the proper precautions. When working with fiberglass you should be wearing:

  • Goggles
  • Gloves
  • Respirator
  • Painters suit (to keep the itchy dust off)
  • Working in a well ventilated workspace (preferably outdoors)

Take everything that you were wearing and throw it away. You really don't want to wash clothing that has been exposed to fiberglass, or dust from sanding fiberglass with other clothes in the washing machine because you might just end up spreading the itchy strands all over your once safe clothing.


Aside from the resin and fiberglass itself, some other tools are required.

  • Power sanders
  • Sanding pads and sandpaper
  • Mixing bowl (a disposable plastic container is fine)
  • Mixing stick (piece of scrap wood)
  • Measuring tools - pumps that dispense the resin and the hardener in the correct ratio or a measuring cup
  • Scraper
  • Disposable paint brush
  • Rags
  • Solvents for cleaning

Useful places for more information and supplies:

There are tons of good tips already published right here on Instructables. Here are just a few.

Mixing Polyester Resin by Trebuchet03
Handy Tricks (for removing fiberglass from skin) by TimAnderson
Epoxy Laminating Systems by freemanmfg
Fiberglass Tools and Tricks by unclesam
Using Paper, Resin and Fiberglass to be the Master Chief! by Dr.Professor_Jake_Biggs

Here are some companies that sell fiberglassing supplies.

US Composites has a wide range of resins.
West System is a good source for marine resins and fillers.
System Three has tons of marine application resin
TAP Plastics has many locations in the bay area and stocks a wide range of resin's fiberglass mat, and the tools needed to work with it all. They also have some informative .pdf's located in their FAQ section.

Check out your local store to get an idea of what you'll need and then see what's online. If you don't happen to be next to a major retailer of fiberglass supplies, it could be a good idea to order online since the price of resin varies wildly depending on where you get it.

Step 2: Sand Down/Out the Bad Fiber Glass

The first step in fixing a crack is to sand out or down the broken fiber glass. It comes away pretty quickly if you use an abrasive pad on a power sander or angle grinder. Be careful though, you can easily take off too much material and grind into something that you didn't want to.

There is no hard and fast rule that says when you have taken out enough of the bad glass. A good rule of thumb would be to clear out a section of fiber glass a few inches around the original crack.

You'll be sanding through hard little pieces of old fiberglass and epoxy and you certainly don't want to get any of that stuff in your lungs or clothing so now is definitely the time to wear the proper safety gear.

If you are starting a new fiberglass project and not using it to fix a crack then you won't need to do this step.

Step 3: Apply a Thin Coat of Epoxy

The first step is to mix up a batch of two-part epoxy resin. This is used to seal the open foam core of the board. You can use whatever resin system you like - just mix the resin according to the directions on the bottle and make sure you measure carefully. We happen to be using West System 105 epoxy resin.

Some things to think about when mixing resin for the first time are:

  • How much resin will I be using for this step? Small batches are better then big batches since the pot life of most resins will be between 15 and 30 minutes once they are mixed.
  • Are all my project pieces in place?
  • Does this resin need to be thickened for this application? If you are going to be applying the resin on a vertical surface, or are wanting the resin to act more like a filler (like bondo) then you'll probably want to add a thickener agent, like carbosil, or fairing filler as I will refer to it in the next step.

Once things were prepped, we mixed our epoxy resin system according to the directions and applied a liberal amount of the resin with a brush onto the entire area where the inner core foam was exposed and then allowed it to dry.

It's best to lay down this primer coat of resin that will penetrate the project surface before adding in any of the thicker resins that contain fillers so that they'll have something to grab on to.

Step 4: Apply a Heavy Coat of Epoxy With a Filler

Once the first brush on coat of resin was dry it was time to mix up a thicker batch of epoxy and apply it using a plastic scraper. The first thin coat of resin just sealed the foam a bit and gave this second layer a nice base coat to hold on to.

We mixed up the same base batch of epoxy resin as before, but this time added in West System's 410 Fairing Filler. The Fairing Filler is made from very fine grounds of silica. The grinds are so small that they float like bubbles, which means that it's perfect for getting into your lungs - so wear a respirator when working with this stuff.

Add enough filler so that your mixture becomes the consistency of peanut butter (it's really more the consistency of peanut butter on a hot day when you first open the jar and it hasn't been totally mixed and the oily stuff is still at the top, but that might be more detail then required...) The amount of fairing filler that you add in will be up to you and will vary depending on how big of a batch you are mixing. We mixed a couple of tablespoons of the stuff into an 8oz batch of resin.

With the resin thoroughly mixed, take a putty knife, or a simple spreading tool like the one pictured below and work the resin and filler onto the board covering any area that was ground or sanded down.

Take your time and make sure that you've filled in all of your holes. Spread the resin mixture around and get it to be as smooth as you can. It's not imperative to make the epoxy flat like glass in this step, but it will be good practice for the final coat that goes on.

Step 5: Prep the Area and Layout the Design

Once the resin with the filler dried I went back with a palm sander that had a medium grit paper on it and sanded the entire area down as smooth as I could. This is the foundation for the fiber glass to lay down on, so it's important that it creates a seamless transition to the rest of the board.

With the resin dry and sanded it was time to start thinking about where the fiberglass would go, and how much we would lay down. Danny thought about 4 layers of glass cut into different size patches would be good, so that's what we did. Tape off an area thats just a bit larger then the entire space that you are going to work on. Use a pencil to draw right on your surface that's going to be fiberglassed in order to see what size pieces of glass you will need. We laid down a larger area of fiberglass then where the fiberglass was located in order to strengthen the entire bottom of the board to protect against future cracks.

General recommendations for planning out your fiberglass project are to use multiple small pieces of fiberglass, rather than one big one to strengthen your surface and to overlap them with each other by about 1 or 2 inches. Usually somewhere between 1 and 4 layers of fiberglass will be strong enough for most projects, but you can build up as many layers of fiberglass as you'd like. Just don't let any one round of layering build up to be thicker then 1/8" since you'll want to avoid excessive heat buildup from the resin as it cures - it's an exothermic reaction.

(Just to give you an idea, 1/8" of fiberglass is somewhere around 12 sheets thick, so it's not much of a limitation.)

Step 6: Cut the Fiber Glass and Mix More Resin

Once we figured out where we wanted the fiber glass to go, it was time to go cut out some pieces that we could use to lay down. We used woven glass mat, which means that the glass is woven with pieces of glass threads that are woven together like a textile. This type of fabric is stronger, but more expensive then chopped mat.

Rather then cutting the rectangular pieces out on those lines, cut across them at 45 degrees so that the fibers don't unravel. In other words, turn the the fiber glass so that you see lots of x's, rather then lots of +'s all over it. Then cut out your pattern as per usual.

The cuts don't need to be terribly precise in this step - it's all going to get sanded smooth so it's fine if the sheets have some rough or partially fraying edges.

Before continuing, it's best to make sure that everything is prepared and ready to go. The final part of this step is to mix the resin and once the hardener is added, the clock is ticking.

With everything ready, we mixed up a batch of epoxy laminating resin from System Three, but again, you could use any kind of laminating resin suitable for working with fiberglass in this step.

Step 7: Lay Down the Fiber Glass

Take the first piece of fiberglass that is going to be laid down and place it on the surface. Use a brush loaded with the laminating resin and begin to paint a thin layer of resin over the fiberglass. As you paint more resin onto the mat, it will turn translucent and lay down nicely on the surface below. Work resin from the center of the mat out to the sides. This process is called "wetting out" the fiberglass.

Once the fiberglass has been thoroughly saturated with resin switch from the brush to the scraper, and just as you would apply spackle to a wall, use the scraper to apply a thin even coat of the resin all over the fiberglass. Work from the center of the piece to the perimeter of your work. Be mindful of any bubbles or folds in the fiber glass as you are trying to lay down the layer of fiberglass as smoothly and evenly as possible.

Don't worry about the glass extending beyond the edges of your project surface. The extra bits can be sanded off easily once the resin dries.

Ribbed roller tools make this process even easier since they are great at getting air bubbles out of the fiberglass and work really well at getting the fiberglass to lay down.

Once the first layer was down we began to layer more fiberglass on top. Each successive layer won't require as much resin as the previous layer did since there is usually excess resin made available from the scraping process.

You can lay down as many layers as time permits before your resin becomes too hard to work with (between 15 and 30 minuted depending on your system). If you're planning on laying down many many layers, think about mixing multiple smaller batches of resin rather then one giant batch and rushing through the work.

When all the layers are down and the resin is spread evenly and completely throughout the fiber glass it's time to let things dry thoroughly. This can be hour, or it can be overnight depending on temperature and what resin system you are using.

You can control how quickly polyester resins cure (within a reasonable range) by changing the amount of MEKP that is added to the resin. Epoxy resins cure at different speeds depending on temperature, but generally speaking, the only way to manipulate drying times reliably is to buy resin systems that are specifically formulated to go off in either long or short amounts of time.

Step 8: Sand the Fiber Glass and Resin Layers

Once everything has had a chance to dry overnight it's time for another round of sanding.

Grab the power sander and set it up with a medium grit sandpaper and sand down any bumps or ripples in the dried resin.

Make sure you don't go too deep, since you'll be sanding away the fiberglass that you just worked so hard on laying down.

Use the sander to make everything as smooth as possible. That includes gently feathering the resin at the edges of the area where the resin meets the original project surface. This is also a good time to trim off the excess fiberglass that overhangs the project. Just lightly sanding the rails or sides of the project surface will take off the brittle resin filled ends of the fiberglass.

Step 9: The Final Layer(s) of Resin

At this point the board should be sanded fairly smooth, but more then likely it won't be perfect. Some of the strands of fiberglass may have come unraveled when you laid the fiberglass down and created raised vein like lines on the work surface. Or, perhaps, the lattice texture of the fiberglass will be too rough of a surface for a finished product. Whatever the case may be, it's likely that the project will require one or two more final coats of resin and filler, and final sanding to get things perfectly smooth and seamless.

For the final coats we went back to the thick West Systems two part marine resin mixed with West System's 410 Fairing Filler to thicken the mixture up. The resin gets applied in the same way it did before in step 4.

Apply a dollop in the center of the piece and work it out to the sides applying it like spackle on a wall. It should flow nicely, and if it's not too cold outside it should coat the top of your work surface evenly.

Use two thin coats to finish up rather then one thick coat if the surface is too rough or if you are trying to patch an area of a functional surface (the bottom of a kite board for example).

Let the layers dry and sand one before applying the next. Switch from medium grit paper to fine grit paper for the final round of sanding and be careful not too over sand or gauge into the project with the edge of the sander. Keep the sanding pad flat on the project and don't be tempted to push harder or sand with the edge.

Step 10: Check for Smoothness

Use the edge of something straight to check and see how smooth and level your work is. Hopefully the four layers of fiberglass that you laid down should be only very slightly raised above the rest of the project surface and should create an almost imperceivable bump in the area that got repaired. If there is more a bump or raised section, then you either put down too much resin, laid down too many layer of fiberglass, or didn't sand your work in between coats sufficiently.

I was surprised at how thin the repair was. Even after laying down four layers of fiberglass and 5-6 layers of resin it was still just a tiny bit higher then the surface of the original kite board.

The final product wasn't visually very impressive since the original decal was sanded off and replaced with just a big old swatch of tan epoxy, but the crack was perfectly mended and the board is likely to be significantly stronger then it was when it was originally made in the factory.

Step 11: Cleanup

Cleaning up resin is not the easiest thing to do, and as a result, it's generally best if you try to exercise caution while working with it so the mess can be kept as small as possible.

Rollers, stirring sticks and paddles can all be rinsed and or cleaned with a solvent of choice. Acetone works well, as well as lacquer thinner, paint thinner and denatured alcohol. Some of these solvents will work better then the others and aside depending on your resin system and how long the resin has had time to cure.

Gloves and rags should be unrolled or folded and laid flat to try outdoors. Most solvents are highly flammable and since epoxy generates heat as it cures it's best not to simply throw your rags in the trash bin while they are wet.

Mixing containers can be cleaned by first emptying out as much resin as possible onto an expendable surface (say a piece of cardboard) and allowed to cure. Whatever is left in the mixing container can be cleaned along with the rest of the tools using a solvent. An alternative method for cleaning containers is to allow the remainder of the resin to dry in the container. Once it's dry, you can squeeze the plastic container causing the thin layer of resin to crack and fall out.

Wear gloves throughout the entire cleanup process as solvents remove the oils from your skin and make you more susceptible to absorbing the harmful agents in the resin into your body.

Once the cleanup is complete, should resin be needed to be cleaned from your skin, wash with a cleaner like Gojo or Fast Orange which won't remove your skin's protective oils. Using solvents on direct skin is not recommended and will actually help the resin be absorbed through your skin faster.

Dispose of your rubber gloves and painters suit and wash your clothing in it's own batch so it doesn't get itchy fibers all over your other clothes.

If you have more tips on how to work with fiberglass, please add them as comments to this Instructable so I can add them into this guide.