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Wondering How Fiberglass Pools Are Made? Let's Jump Right In!

Best Fiberglass Pool Construction

Experience The Sandwich!

Food is always fun right?  So let's think of a fiberglass pool shell as a huge a club sandwich from the local deli.  A club sandwich is a composite, which basically means it's made up of different materials like bread, turkey, ham, bacon, lettuce, tomato, mayo, etc.  The role of each layer is to add flavor and texture to the whole.  

Such is the case with a fiberglass pool, which is also a composite.  It's manufactured in multiple layers, each layer serving to maximize the strength and durability of the entire structure.  In other words, a fiberglass pool is like a sandwich of strength.    

To take the food analogy a step further, perhaps we could compare the qualities of a great sandwich to the qualities of a great fiberglass pool.


What Makes a Sandwich Great?  

First, you have to have great ingredients and they have to be fresh (especially the bread).  The amount of each ingredient is also important (especially if pickles are involved).  And one could make the argument that the order the ingredients are added can make a big difference too. Finally, attention to detail can't be overestimated.  

In short we could say that the right ingredients, in the hands of the right people, and assembled in the right way are all that one needs to make an epic sandwich.     

What does this have to do with fiberglass pools?  Everything!    

Just as all sandwiches are not created equal, neither are all fiberglass pools created equal.

But before we get into the science of our fiberglass pool manufacturing, let's pause for a moment and explain how we came to build them the way we do.

The River Pools / Thursday Pools Story

At River Pools, we have installed a little over one thousand fiberglass pools since 2001.  We have used this experience to teach everything we know about fiberglass pools to the world and have developed a reputation of trust across the country with homeowners and pool builders alike.  Consequently, our website became one of the most highly trafficked swimming pool sites in the world, however we were limited to only servicing clients on a regional level (VA, MD, and WVA).  There was a burning desire within us to find a way to reach all the dealers and homeowners who knew and loved us, but who we could not serve.  

Introducing Thursday Pools

Thursday Pools, with over 35 years of extensive experience in the composites industry, has worked with a variety of products ranging from boats to race car bodies using materials such as carbon fiber, kevlar, and fiberglass.  As an established fiberglass pool manufacturer, they are seen as thought leaders in the industry and have an impecable reputation for producing the highest quality pools.

River and Thursday Pools have united to form the joint company River Pools Powered by Thursday Pools Manufacturing.  This partnership sends a shockwave throughout the pool industry as the power of education and world class manufacturing combine to reach across the country.  

How Did the Composite Sandwich come into existence?  

Through long established relationships with the chemists at companies like Interplastics and Owens Corning, Thursday Pools has done extensive lab testing such as Finite Element Analysis (FEA) to determine the best possible way to build a fiberglass pool.  The result is a combination of processes and materials that compose most advanced fiberglass pool in existence: Our Composite Sandwich of Awesomeness.

Just to be clear, from here on when "we" or "our" is used, it refers to River Pools Powered by Thursday Pools, not just River Pools.  You can read more about the partnership here.   

Okay, time to learn about the Sandwich!   

9 Layers of Strength

How Are Fiberglass Pools Made?Layer 1: GEL COAT

The first layer of our composite sandwich is called gel coat, which is the colored surface of the pool.  It adds beauty, and being smooth and non-porous, is what gives fiberglass pools a low maintenance advantage over other types of pools.

Our gel coat is 28-30 mils thick, about the thickness of ten sheets of paper, and is applied with special spray equipment that mixes the colored gel coat with a liquid called initiator or catalyst.  The blending of the gel coat and catalyst initiates a chemical reaction causing the gel coat to transform from liquid to solid.  For simplicity we call this curing.  To be specific the gel coat does not “dry” it is a chemical reaction called polymerization.   Technically polymerization is a process of reacting monomer molecules together in a chemical reaction to form polymer chains, or three-dimensional networks.  Now you see why we call it curing.  The objective being a properly cured, or hardened, gel coat in the end.  We’ll see how the other phases of manufacturing affect how the gel coat cures later.    

We apply the gel coat in three passes, each roughly 8 mils thick.  Why do we do it this way?  Because building up the entire gel coat thickness all at once prevents the gel coat from de-gassing, which is a process whereby gasses produced from the reaction are released.  If these gasses are trapped, they could produce tiny holes in the gel coat, called porosity.  Applying too much gel coat at once could also result in sags and drips in the finish.  

It’s important not to apply too much gel coat at once, but it’s also important that the entire gel coat layer not exceed 28-30 mils of total thickness.  With gel coat, thicker is not better, as it results in less plasticity and increases the risk of micro-cracking, which is purely cosmetic. No one wants to deal with an issue that is preventable from the start.   

Gel coating requires the most skill of all phases of fiberglass pool construction.  Our gel coat specialists, all trained and certified to our standards, check the thickness of the gel coat constantly throughout the spray process with a mil gauge.  They also inspect the spray equipment before spraying each pool and this, along with the thickness at multiple passes, is recorded on our quality control sheet for every pool we manufacture.  

Our technicians also do not stand or walk on the mold during the first five phases of the build process.  Walking on the mold, although a common practice in the fiberglass pool manufacturing industry, is not a best practice as it increases the risk of problems such as contamination from shoes and distortion of the pool surface and supporting layers.  

Using an appropriate scaffold system, although more inconvenient than walking on the mold, insures that every pool leaves our plant with the best finish possible.

Within an hour of being sprayed, the gel coat is set up enough for the second layer of our Composite Sandwich of Awesomeness.  

Related Articles:

Can Water Behind A Fiberglass Pool Cause Osmotic Blisters Or Damage The Gel Coat Surface?  


Even though gel coat is smooth and relatively non-porous, on a microscopic level it does allow water molecules to pass through and contact the layer behind it.   If that molecule is allowed to come in contact with any contaminates, it will react causing what are known as osmotic blisters to form on the surface of the gel coat.    

To prevent this from happening, we apply the second layer of our Composite Sandwich of Awesomeness called our 100% Vinyl Ester Barrier Coat.  Vinyl Ester Resin is an extremely pure, high grade resin that serves as a water barrier, protecting the gelcoat from blistering.  Think of using mayonnaise as a barrier to prevent your bread from getting soggy, that’s kind of the idea.    

In the following video Bill Khamis, 35 year composites industry vetran and partner here at River Pools Powered by Thursday Pools Manufacturing, explains why we use our Vinyl Ester Barrier Coat as oppossed to a traditional Vinyl Ester layer combined with chopped fiberglass.  Bill also outlines several other unique advanatages of the VE Barrier Coat that truly set our product apart from the competition.  

Our barrier coat is spray applied in a similar process to that of the gel coat to a thickness of 16-20 mils thick.  This process is in stark contrast to most manufacturers throughout the industry who, instead of spraying a pure vinyl ester barrier coat, apply vinyl ester resin in conjunction with a chopped fiberglass filler.  Assuming this is actually a high grade resin and not a low grade blend, this is a proven method to prevent osmotic blisters.  However, there are several disadvantages to using  a chopped fiberglass vinyl ester resin layer as opposed to a 100% Vinyl Ester Barrier Coat with no fillers.

First, remember we mentioned the importance of gel coat getting an adequate cure?  What drives the cure of composites is heat.  As the chemical reaction between the base resin and initiator occurs, exothermic heat is produced.  This heat advances the reaction and specifically causes molecules to cross-link with each other, forming a chain of strength.  The right amount of heat results in better cross-linking.  A composite that is properly cross-linked is called a properly cured part, which means it has reached it’s highest strength potential. .  

The beauty of our Vinyl Ester Barrier Coat is that it helps drive the cure of the gel coat.  Because it is spray applied, our trained technicians can apply it the very moment the gel coat has set up enough to receive it.  The Barrier Coat then generates exothermic heat from it’s own reaction thus further driving the cure of the gel coat.  It’s kind of like adding a hand warmer to your sleeping bag when camping in the winter.    

This is not the case with a chopped vinyl ester resin layer.  Chopped fiberglass, when applied, has a lot of entrained air which has to be removed with a small roller.  Because this process is much more invasive to the newly sprayed gelcoat surface, more time is required between applications.  While waiting, the gelcoat loses it’s own exothermic heat as it is released into the air, plus it’s not benefiting from the additional heat from the Barrier Coat reaction.  It’s kind of like leaving your sleeping bag partially unzipped.    

Furthermore, because our Barrier Coat is applied while the gel coat is still tacky, there is a superior chemical and mechanical bond between layers.  

The Barrier Coat also prevents what is known as “print through”.  This is where the outline of the glass fibers from the backing layer show through the gel coat and become visible.  Because our Barrier Coat is sprayed without any glass, it provides an increased buffer between the gel coat and strength layers resulting in a better finish.       

The Barrier Coat also provides superior blister protection than that of a standard Vinyl Ester Choped Fiberglass layer.  This is not our opinion but stated clearly by the manufacturer of Our Barrier Coat.  

In the following video, Bill Khamis answers the question, "Can water on the backside of a fiberglass pool cause the gelcoat on the inside of the pool to blister?"  

Basically, our Vinyl Ester Barrier Coat acts serves as the perfect water barrier from both sides of the pool, and becuase it does not have chopped fiberglass, it's virtually impossible for water to "wick" through strands of fiber and reach the gelcoat surface of the pool. 

Again, using a chopped fiberglass vinyl ester barrier coat is not a bad practice.  However, decades of experience compels us to do everything in our power to ensure the best cure, the strongest possible bond between layers, and the best finish possible across the entire pool shell.  Even though our Vinyl Ester Barrier Coat is more labor intensive and expensive, it’s a sacrifice we are willing to make to put the  “Awesomeness” in our “Composite Sandwich”.  

After approximately 60 minutes, the Barrier Coat is set up and we are ready for our next phase.  But first, it’s important to understand the term “laminate schedule”.  

What Is A Fiberglass Pool Laminate Schedule?

Once the gel coat finish is applied and protected by the Barrier Coat, we are ready to begin adding strength to the pool by adding multiple layers of fiberglass.  All of the layers together make up what is known as a laminate schedule, which outlines the type and order that these materials are applied.    

Some laminate schedules are more complex than others, and they vary dramatically from manufacturer to manufacturer, but make no mistake, it’s the laminate schedule that determines the strength and durability of the pool.  For years, we were on a quest to find the strongest and most durable laminate schedule.  Our “Composite Sandwich of Awesomeness” may sound funny, but as we discussed earlier, it is the result of years of rigorous laboratory testing.   

Our “Composite Sandwich” is not just the PB&J of the fiberglass pool industry, it is a product of years of evolution.  Every layer weighed, measured, and found worthy of it’s rightful place in our laminate schedule.  

Related Articles: 

4 Fiberglass Pool Manufacturing Problems Solved: Understanding Our Barrier Coat



Fiberglass Pool Layers

In fiberglass pool production, fiberglass is basically composed of three materials: roving, resin, and initiator(typically called catalyst).  Roving is the actual glass fiber product and is formed by melting silica and other products into a liquid and extruding it into tiny continuous fibers.  Roving has incredible tensile strength, which means it is resistant to forces that try to pull it apart.  This is one of the reasons fiberglass is so strong.  The two predominate types of roving used in the production of fiberglass pools are gun roving and woven roving.  

Chopped fiberglass, the first strength layer of our “Composite Sandwich” is composed of gun roving, which begins as a continuous stand of fibers bundled together and coiled into a fifty pound roll.  The roving is pulled from the roll to the tip of a chopper gun where the strand is cut into approximately 1” lengths by a wheel of rotating razor blades.  As the gun chops the fibers, they are blown onto the surface by resin that is simultaneously sprayed from the tip of the gun.

Our gun roving, Advantex® E-CR® is produced by Owens Corning, the founder and largest producer of glass fibers in the world.  It is the highest quality glass fiber on the market today.  E-CR glass is lab tested to far outperform traditional roving both in terms of strength and chemical resistance, which is what the “CR” stands for.    

The resin used in the production of fiberglass pools is called a thermoset resin, which remains in a liquid state until combined with an initiator, or catalyst, that sets in motion the cross-linking reaction we mentioned earlier.  Because the thermoset resin is applied in a liquid state, it can fully saturate the roving.  When fully hardened, the resin and roving join to form fiberglass, a product with one of highest strength to weight ratios of any man-made material.

Are All Resins Created Equal?

Back to the sandwich analogy, which lunchmeat would you prefer: turkey sliced right off an oven roasted breast, or processed “turkey” that is loaded with filler. 

And this brings us to one of the major differences between fiberglass pool manufacturers, the quality of resin used.  The quality of the resin is determined by the purity of the resin.  The use of filled resin does save money, but at the cost of sacrificing strength.  We never have and never will use filled resin.  The result is a stronger pool for your backyard.

Why Do We Use Chopped Fiberglass For First Strength Layer?

Chopped fiberglass has several advantages that make it ideal for use directly behind our Vinyl Ester Barrier Coat.  First, it’s important to understand that when applying fiberglass the objective is to properly consolidate the material through rolling the material out to remove any air that is trapped in the laminate.  It’s extremely important that the layer behind the Barrier Coat has no voids or air bubbles that would prevent it from coming in perfect contact with the entire surface.  We use chopped fiberglass because as it is applied, it forms to the contour of the pool and it relatively easy to roll out.  It’s also important that this layer is applied and completed as soon as possible to obtain a good chemical and mechanical bond and to help continue to drive the cure of the previous layers.  Any alternative fiberglass layer would take substantially longer to apply.      


The fourth layer of our Composite Sandwich of Awesomeness is yet another layer of chopped fiberglass.  Why would we do this again?  Great question!

As we discussed, each layer serves a specific purpose and the purpose of this second chopped fiberglass layer is to add thickness, as well as strength, to the pool shell.  You may be wondering why we separate the two chopped layers into two different applications.  

Two words: heat distortion.

We discussed how exothermic heat can be good for the curing process as it drives the cross-linking process.  Too much heat can actually be a bad thing.  If the thickness of the pool is built up too rapidly, too much exothermic heat is generated and heat distortion can occur.  This is when the tremendous heat generated by the crosslinking process causes the pool shape to distort resulting in waves or ripples to appear in the pool shell.     

Applying too much chopped fiberglass at once can also cause the glass to sag before it has a chance to harden.  This is why we take the extra time and energy to administer these chopped fiberglass layers in two applications.  


We might compare the the fifth layer of our laminate schedule to the meat of the sandwich.  Woven roving is different from chopped gun roving in that the fibers are long  and woven together to form a fabric, as opposed to being chopped and randomly oriented as it is applied.  When it comes to glass fiber, the longer the strand the higher the tensile strength.  Consequently, fiberglass with woven roving is much more flexible than chopped fiberglass.  Think of a spider web, how when force is applied, either from the wind or from something coming in contact with it, all the long strands elongate and work together to help it hold its original shape.  

Woven roving is applied by hand.  The process is to first spray resin on the surface, enough to saturate the backside of the material when unrolled.  If enough resin is not applied from the beginning, there will be dry spots throughout the laminate and the strength of the pool will be greatly compromised.  Then the roving is unrolled onto the wetted surface and massaged to form to the surface of the pool.  Then more resin is applied to the unrolled woven roving to “wet out” the entire laminate.  The surface is rolled throughout the process to insure there are no voids or air pockets.

We apply woven roving to the entire floor of the pool as well as throughout all of the steps, benches, and curves to insure the pool shell is as strong and flexible as possible.   

Why not use all woven roving?  

We certainly could, and we would actually use less resin if we produced pools in this manor because fiberglass with woven roving has a higher glass to resin ratio than chopped fiberglass.  However, we don’t feel that this would produce the best fiberglass pool, mainly because more flexibility is not always desired.  Fiberglass pools are lifted and manipulated in incredible ways.  Consequently, some rigidity is needed.  Our laminate schedule is designed to strike the perfect balance of flexibility and rigidity.


Following our layer of woven roving, another layer of chopped fiberglass is applied to the pool shell.  By this time, the structure of the pool has become substantial enough for our fiberglass technicians to walk on the pool as it’s being built.  This third layer of chopped fiberglass is critical because it provides the bedding for the seventh layer of our Composite Sandwich of Awesomeness, our Honeycomb Core.  

Composite Sandwich of Awesomeness


Have you ever noticed that a honeycomb is composed of a series of hexagons?  Bees could build their homes using any formation, why do they use this shape?  Maybe because the hexagon is the most efficient use of space in all of nature.  As hexagonal cells interlock they form a rigid structure with an abundance of open space.  Just the perfect shape for a honey bee, as well as adding a lightweight and structural support to a fiberglass pool.  

Our Honeycomb Core is a series of lightweight structural honeycomb panels that, sandwiched between layers of laminate, dramatically strengthen the pool shell in strategic areas.  We use the honeycomb core in several distinct ways.  First, it is used as vertical ribs spaced along the pool walls.  This adds rigidity and prevents wall bugles from developing.  

We also integrate our Honeycomb Core beneath the entire area of every step and bench seat in the pool.  This not only adds strength to these critical areas but also provides solid feeling steps and benches.  

Do you know what causes the most stress on a fiberglass pool?  

The most stressful time in the life of a fiberglass pool is typically when it is lifted and moved.  And what part of the pool is stressed the most?  It’s the top, or the coping of the pool as we also call it.  It’s for this reason that we also add our Honeycomb Core around the entire top coping of the pool shell.  Does it require more time and materials?  Sure, but it's worth it to know that whoever is handling our pools, they are built to take it.      

Why is the Honeycomb Core placed where it is in the laminate schedule?

Independent laboratory testing of our samples, as we discussed earlier, has shown that placing our Honeycomb Core as near to the center of the laminate schedule as possible exponentially increases its strength benefits.  We don’t call it a “Composite Sandwich of Awesomeness” for nothing!


This layer of chopped fiberglass seals the Honeycomb Core in place and fixes its location near the middle of the laminate schedule.  This fourth layer of chopped fiberglass also provides additional strength and thickness to the pool shell.  Once the entire pool is “chopped” and rolled out, we can begin our ninth and final layer.


So, we probably began making our Composite Sandwich of Awesomeness around five in the morning when our first layer, the Gel Coat, was sprayed.  Shortly there-after we applied our 100% Vinyl Ester Barrier Coat, followed by our first and second layers of chopped fiberglass, then our woven roving layer was rolled out.  We followed that with the fourth layer of chopped fiberglass, then our Honeycomb Core.  Then we applied another layer of chopped fiberglass, our fifth, and now it’s probably around 4 p.m. and we are about to apply our final chopped fiberglass layer of our composite sandwich.    

Once this is applied and rolled out, the pool will be allowed to cure for the next 24 hours before it is removed from the mold.  Once demolded, the edges of the pool are trimmed and sanded nice and smooth and the pool is ready to deliver to some happy family’s backyard.  Maybe yours? 

In the following video, we show the three step process involved in the creation of a new fiberglass pool design: the pattern, the mold, and finally the construction of the pool shell.  Enjoy!


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