Fiberglass 101 -- The Basics

First of all, let me explain exactly what "fiberglass" is. It's a composite material, meaning that it's comprised of two (or more) different materials mixed together. Typically, fiberglass consists of a matrix of glass fiber, saturated with a polymer resin. The glass fiber has good tensile strength, but is flexible (like cloth). The resin locks the glass fibers in place, to keep them rigid. The end result is a strong, relatively lightweight compound, that is resistant to moisture and chemicals.


Making fiberglass involves just as much art as science. While science creates the high-tech materials that we use in fiberglass molding, it often takes the vision of an artist to form it into a beautiful shape. And while the directions on the can will tell you exactly how to mix the resin, making a plug or a mold will often be a "seat of the pants" venture. There is no set-in-stone formula for making a fiberglass mold, or a fiberglass part. So you have some freedom for artistic expression. However, there are some general guidelines you will need to follow.

Also keep in mind that the fiberglass layup is only a small part of the whole process. Most of the work involved in making a fiberglass part is actually in the prep work and finish work. I often tell people that making fiberglass is about 40% prep work, 20% fiberglass layup, and 40% finish work. Be prepared to do a lot of grinding and sanding.


There are two main types of resins used in automotive applications: polyester and epoxy. Both have their advantages and disadvantages. See the list below.


Low priceAdjustable curing time
More brittle than epoxyShrinks when curingNot as resistant to water and chemicals as epoxyHorrible odor


Doesn't shrink when curingNot as brittle as polyesterMore resistant to water and chemicals than polyesterVery little odor
High priceCuring Time isn't adjustable
I personally prefer epoxy resin, because of its superior qualities. In my opinion, it's worth the extra cost. But for automobile body parts, polyester resin will work too. My suggestion would be to try using both types of resin, and see which one you like better.


Polyester and epoxy resin are fundamentally different in the way they are mixed and applied. Polyester resin uses a catalyst to harden it (called MEKP), which is mixed in a very low ratio (a few drops per ounce). But with epoxy resin, the hardener is mixed at a much higer ratio (which could be 1:1, 3:1, 4:1, etc depending on the type of epoxy). It's also important to note that while you can vary the curing time of polyester resin by increasing or reducing the amount of hardener, you cannot do that with epoxy. You must mix the epoxy and hardener at a predetermined ratio. Otherwise, the epoxy will not properly cure.

Also, most polyester resins have a little bit of wax mixed in. As the resin is curing, the wax rises to the top and seals out the air (to help it cure). This causes two "gotchas". First is the fact that the wax rises. This means that the exposed surface needs to be facing up. If the exposed surface is facing down, the wax won't come to the surface. And the resin won't properly cure. Secondly, you will need to remove the waxy film after the resin cures. Otherwise, subsequent layers of fiberglass, or paint, will not adhere to the resin.

Another thing to keep in mind is that resin is exothermic, meaning that it creates heat as it cures. The heat aids in the curing process. This means that the temperature of your working environment will affect the curing time of the resin. This also means that you can use heat (or cold) to control the curing time. For example, I will sometimes use a cold pack to slow down the curing of my epoxy resin. And when I want something to cure quickly, I will let it sit in direct sunlight. Heat lamps can also be useful.


Glass fiber is the heart of any fiberglass project. There are two main types, cloth and mat. The glass fiber cloth is quite literally a cloth made of woven glass fibers. The glass fibers are typically woven in a cross-hatch pattern. This makes the glass fiber cloth relatively dense, which increases the strength and reduces the amount of resin needed to fill the voids. Glass fiber mat, on the other hand, is basically just a bunch of short strands piled up at random. They are not woven together. As a result, the mat is not as strong as the cloth. And there is more open space between the strands, requiring more resin to fill the voids.

The bottom line is that glass fiber cloth will yield stronger, lighter parts than glass fiber mat. And since the glass cloth needs less resin, it will also be less brittle. In my opinion, glass mat is really only suitable for building up thickness, or for parts where expediency is more important than strength or weight.

One more thing: if you're using glass fiber mat with epoxy resin, make sure the mat you use is compatible with epoxy resin. While almost all glass fiber cloth is compatible with either epoxy or polyester, glass fiber mat is often formulated specifically for use with polyester resin. Check the packaging to be sure.


The resin and glass fiber are not solid until after the resin cures. When you mix the resin, it's a liquid. And the glass fiber is flexible. The wet fiberglass will sag, or even collapse, under its own weight. For small jobs (patching cracks and holes, filling gaps and seams), this isn't much of an issue. But for anything bigger, the fiberglass will need to be supported. This is where molds come in. The mold gives the fiberglass something to rest on, and a shape to conform to, until the resin hardens.

When building a fiberglass mold, there are several things to take into consideration. Some examples:

 -- Will the mold be used only once, or to make several parts? If it's a single-use mold, it only needs to hold together long enough for the fiberglass to harden. But if it will be used to make several parts, it must be sturdy enough to withstand the rigors of having fiberglass parts pried out of it several times.

 -- How will I remove the fiberglass from the mold after it hardens? For simple shapes, this is pretty easy. But for complex or convoluted shapes, it is quite possible to get the fiberglass trapped in the mold, or vice versa. In order to avoid that, you will need to either make a multi-piece mold that can be pulled from the fiberglass part piece by piece, or make several smaller fiberglass parts that can later be joined together to form the complete structure.

 -- Is my mold material compatible with my fiberglass resin? I'm sure you've heard that polyester resin melts styrofoam. It also attacks some types of plastics. However, it does not affect urethane foam. Epoxy resin doesn't affect any type of plastic, that I know of. Also, keep in mind that fiberglass resin can get pretty warm as it hardens. So make sure to avoid using mold materials that are susceptible to heat. Fiberglass curing in the sun can get hot enough to hurt if you touch it. And I've occasionally seen resin get hot enough to melt the bottom out of a plastic cup.


Sometimes, you can't just build a mold and start making your fiberglass parts. You may want to copy an existing part. Or maybe making the mold from scratch would be problematic in some way. In these cases, you would use what's called a "plug". A plug is a mock-up of the part you want build from fiberglass. After you build the plug, you make a casting of it. The casting becomes your fiberglass mold.

You can build a plug out of just about anything. Use whatever works to get the desired shape... scrap wood, styrofoam, plaster of paris, sheet metal, whatever. I've even used my car as a plug.

Once you finish building the plug, you need to prepare it for the casting of the mold. This means that if you used foam or wood (or any porous material, for that matter), you need to seal it. I've used packing tape, duct tape, and garbage bags to good effect. Some people like to use aluminum foil. I've also used polyurethane and even epoxy resin on occasion. The idea is to make the surface as smooth as possible. Then you use a mold release agent to be sure the casting won't permanently bond to your plug.

It's pretty common to cast the mold from fiberglass. The process is pretty similar to making a fiberglass part in a mold. However, you want the casting to be much stronger than the fiberglass parts it will be used to make. This means that your casting will be much thicker than the fiberglass parts you plan to make with it. You might also need to incorporate a reinforcing structure into the casting. If you do decide to reinforce your mold, it's best to do this before removing the mold from the plug. That way, you can be sure the mold retains the correct shape.


Hopefully, now you will have a basic understanding of what fiberglass is and how to work with it. As with any skill, you will need hands-on experience. With experience will come increased skill and confidence. In the meantime, here is some more valuable reading material.

What is Fiberglass?Fiberglass 101Fiberglass for NewbiesFiberglass FAQFiber Glast Learning CenterDo-It-Yourself Guide to Polyester Plug and Mold Building
If you find any factual errors in this article, please send me an email or PM. Thanks.[/list]
8 part Step By Step Guide To Molding Fiberglass. A simple introduction to fiberglass mold construction; explained using a model aircraft cowling. From plug, to mold, to finished fiberglass part.

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