What is Polyester Resin – Complete Guide with Tutorial

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Especially if you have already worked with epoxy resin, you know about the positive properties of this material. However, it has two disadvantages: It’s very expensive and it has to be mixed to the milligram. Especially for beginners this could be a deterrent. Therefore you might want to look for alternatives. Polyester resin could be one of them. In our article we will tell you all the positive and negative properties of this interesting material. We also compare it with epoxy resin and show you what to look for before you buy polyester resin products



What is Polyester Resin?

Unsaturated polyester resin, similar to epoxy resin, is a material that is used, among other things, for the production of various components in model making, but is also used in boat building or in the construction of swimming pools and ponds.

With the help of polyester resin, various fibre composites can be produced, which are known under the name Duroplaste. In contrast to thermoplastics, these are neither formable nor weldable after curing.

Polyester resins are divided into fast and slow curing. In between there are a multitude of gradations. This so-called reactivity has a serious influence on the chemical as well as mechanical properties of the finished workpiece, as it influences the cross-linking during the curing phase.



Properties of Polyester Resin

The properties of this synthetic resin are extremely variable, since not only the areas of application but also the incorporation of various filling materials and colorants play a role. In general, it can be said that polyester resins are excellently suited for bonding with glass fibres because of their high penetration properties. After curing, the workpieces are weather and water resistant. They also stand up well to diluted, non-oxidising acids and aqueous salt solutions.

Tip: You can also buy food grade polyester resin. In case of doubt, you should ask the manufacturer or consult the technical data sheet to find out whether the resin you want to use has this property. Food-safe means that the material used does not affect the taste of the food or the release of harmful substances.



What can Polyester Resin be used for?

In principle, there are hardly any limits to this. For example, it can be used to create positive and negative forms that can be used for a wide variety of objects. It is used for boat building as well as for the casting of bumpers or gutters. The resin is also suitable for the repair of plastic parts.

Clear Polyester resin is excellent for sealing swimming pools, jacuzzis or garden ponds, but also for the construction of various weather-resistant outdoor objects.



Epoxy vs Polyester Resin – What’s better?

Polyester resin has some outstanding advantages compared to epoxy resin. However, there are also some disadvantageous properties:




  • Polyester resin is much cheaper to purchase. Especially if you want to process large quantities, a price comparison here could be worthwhile.
  • Furthermore, the processing time for polyester resin can be partly determined by the user. You can choose a different time window until curing by adding different amounts of hardener to the liquid polyester resin. It is best to follow the manufacturer’s data sheet. The addition can vary between 1.0 and 2.5 %. The two components of the epoxy resin, on the other hand, must be weighed very carefully, otherwise it will not cure sufficiently or, if too much hardener is used, it will become very brittle.
  • When working with polyester resin you are therefore much more flexible in terms of time during the design phase than when using epoxy resin.
  • You are also relatively free in your choice of reinforcing fabric. You can choose between glass filament fabric, glass fibre mats and glass fibre fabrics. These are needed to achieve greater stability in building components. However, epoxy resin is also suitable for glass filament, carbon and aramid fabrics.




  • When processing polyester resin, a distinct odour is produced, as it usually contains styrene. It also takes a long time for the odour to escape during the drying phase. Therefore it is more suitable for outdoor use. Epoxy resin, however, is also suitable for indoor use, as there is little or no odour when using it.
  • The mechanical properties of polyester resin are also more negative than those of epoxy resin. These indicate how resilient a material is. The epoxy resin is noticeably more resistant to mechanical influences. In addition, epoxy resin is not only suitable for use on roughened surfaces, whereas its cheaper competitor fails on smooth surfaces such as glass or metal.
    Another property is important in boat building: Epoxy resin is much more water resistant than polyester resin.



What to look for when buying?

There are several things to consider when buying it.


  • The glass fibre mats used in laminating and mould making are available in many different thicknesses. The choice here can be very confusing. The same applies to the polyester resin itself. There are many products on the open market and not all of them are suitable for your planned project. If this is your first try with polyester resin, we recommend that you inform yourself thoroughly about the products before you buy.
  • Pay attention with the purchase to considerable manufacturers. Do not buy a product that is offered cheaply, but may then prove to be unsuitable. With products from good manufacturers you are on the safe side. Otherwise, you may have to put a lot of effort into mould making, only to discover that the material does not harden completely.



Materials required for processing

Exactly which materials you need depends, of course, on your project. We will give you a basic overview of the utensils you will need, followed by a brief introduction to the laminating technique and the construction of a negative mold with polyester resin.


You basically need:

  • Polyester resin, suitable for your project
  • Glass fiber mats, possibly in different thicknesses
  • Acetone
  • Possibly priming wax for absorbent or porous surfaces
  • Film release agent
  • A soft sponge or brush
  • A spatula
  • A scale, preferably digital
  • Fine layer resin or gelcoat
  • A venting roller
  • Possibly pigment or another suitable color
  • disposable gloves
  • One dosing syringe
  • scissors and knives
  • Beaker, preferably measuring beaker
  • A respirator mask
  • Stirring rods
  • Possibly putty or plasticine
  • A plastic wedge

Make sure you have all materials ready to hand before you start work.



Step-by-step instructions: Processing Polyester Resin

  • The first step in the processing of polyester resin is a clean working environment. Thoroughly remove all dirt and dust to prevent later inclusions in the material.
  • It is best to sand the surface of your substrate and clean it with acetone to make it clean and free of grease.
  • Afterwards, apply foil release agent to be able to easily remove the workpiece later. If you are working with a porous or very absorbent surface, you should first apply a layer of primer wax.
  • Now mix the polyester resin. Especially beginners should be economical with the quantity. It is better to make smaller portions first. As a rule of thumb, only mix as much as you can use up in ten to fifteen minutes, because after about this time the polyester resin hardens. The exact amount depends on your project, of course.
  • Weigh the two components or calculate the amount of hardener required based on the polyester resin weighed out beforehand. You should mix both components very thoroughly. Remember to also stir the side edges of the cup and the bottom to prevent material from settling there. Decant the mixture two or three times to ensure that it is completely mixed.
  • After stirring and repotting thoroughly, the resin can be used.
  • Now it depends on what exactly you want to achieve for a project. We will introduce you to both laminating and making a mould. When making moulds, please note that negative moulds are particularly suitable for body parts or model making, while positive moulds are mainly used for pool and pond construction.

clear polyester casting resin


Construction of a negative Mould with original Form with clear Polyester Resin

  • First of all, make sure that the master model is very clean and well finished, so that there are no annoying overhangs, corners or noses. This will save you a lot of time during the finishing process, as your form no longer requires any tiresome grinding or milling.
  • First seal joints with plasticine or filler. The plasticine is used by pressing small rolls of it in. You can remove the overhang with a knife or scalpel.
  • Now release wax and film release agent are applied in two layers and then the gelcoat is applied.
  • Try to apply the gelcoat in one direction only, as this avoids annoying air inclusions.
    The cut or torn glass fiber material is inserted. The resin is applied to the glass fiber. Now, wait until it gels slightly. Test if it still pulls threads and then continue working layer by layer, wet in wet.
    When you are finished, allow the mold to cure completely and then remove it using a plastic wedge.

More about about making molds you will find in our how to make silicone mold tutorial.


Laminating with polyester resin

  • The prepared glass fiber mat is impregnated by applying the polyester resin to the substrate using a roller or brush and then laying the glass fibre mat on top. This works either cut or torn, with frayed edges giving you an advantage when working with overlaps.
  • After you have applied another coat of resin, the fiberglass mat becomes soft and can be shaped.

epoxy vs polyester resin

  • When applying, make sure that neither white nor glittering spots remain, because there is too little or too much resin.
  • After the entire mat has been painted, remove the air bubbles by brushing them out with a roller or brush. There are special ventilation rollers for this purpose, which can be used for larger areas.
  • When the material is dry, it can still be painted with a topcoat of polyester resin to make it weatherproof. For this it must be lightly sanded. If you want to paint your object in a specific color, you can add color pigments or color paste to the polyester resin.
  • After work, clean the tools used, to which liquid resin still adheres, promptly with acetone and let it air out well. Once the resin has hardened, you can only sand it down.

polyester casting resinImages: johzio / shutterstock.com


The Curing Phase

How long the finished workpiece takes to harden completely depends on the material, among other things. How long this takes for your used resin brand can be found in the enclosed data sheet of the manufacturer. A factor that significantly influences the duration of the curing process is, besides the material, the size and shape of the workpiece, the temperature. Synthetic resins are available on the open market that cure completely at a temperature of 20 to 25° Celsius, others require a high temperature. These are tempered.



The Tempering

The curing of synthetic resins is a chemical process that can be partially controlled by heat input. The viscosity of the material increases until it reaches the gelling point. Then the resin hardens completely; it changes into the solid state. At higher temperatures, the processing time is reduced because the resin is more fluid and therefore less viscous. The lower the temperature, the longer both processing and curing time will last. In the latter case, it can take up to several days.

Tip: A long, targeted and uniform heat input can improve the structure of the cast component and optimize its mechanical properties. During the so-called tempering process, any air bubbles that may be present are removed, so that any damage to its structure is compensated.



Addition of an Inhibitor

If you need more time to process the resin, you have the option of adding a third, the inhibitor, to the components. As the name suggests, this inhibits or delays the curing process. Especially if you have to process a large amount of polyester resin, you should add an inhibitor.

The following rule of thumb should be used to determine how much of this third component should be added: If you add 0.2% to the component mixture, the processing time is doubled. If you add 0.5 %, it multiplies.

When processing gelcoat, however, the addition of an inhibitor is not recommended, as its structure may suffer.



Coloring Polyester Resin

Just like epoxy resin, polyester resin can also be colored according to your personal taste. However, you must bear in mind that the colors used can have an influence on the structure of the working material. Synthetic resin always consists of a sensitive system of resin and hardener. Adding more to these two components can lead to changes in appearance and function. For example, the added colors have an effect on the viscosity and thus on the curing process. You should therefore pay close attention to which colors are suitable. Examples are specially developed polyester color pastes and color pigments.

Furthermore, it should be noted that the result in the cured resin usually does not quite match the original color. For this reason, always carry out several series of tests before you devote yourself to your actual project. It is annoying if you waste a lot of time, effort and also money because the result does not meet your expectations.


It is possible in three different ways to introduce the paint into the liquid resin:

  • Slow dripping
  • Stir in
  • Dumping

You should pay close attention to when the viscosity of your mixture changes. This process is difficult to determine and requires some experience. Here the saying applies: Practice makes perfect!

By the way, the thicker your resin is, the more difficult it is to distribute the paint evenly in the mixture, whereas a resin that is too thin tends to form streaks.


Safety when handling polyester resin


If you work with polyester resin, you should pay attention to some basic things, because this is a toxic chemical if not already hardened!

Ventilate the room thoroughly, leave the windows open if possible or work outside. Alternatively, wear a breathing mask. Do not allow the material to harden in living rooms, as this causes a strong odour. Do not smoke or handle open fire.

Wear long-sleeved clothing and gloves. Ensure that your skin does not come into contact with the resin. Think about purchasing a full body suit. Safety glasses should also be available.

Also make sure that your surroundings remain as free as possible from polyester resin, because once cured it is very difficult to remove. Protect surfaces that should not come into contact with the resin with foil or thick layers of newspaper. Resin that is still liquid can be quickly removed with cleaning gasoline. The resin cannot be removed from porous surfaces.

If you are working with ketone peroxide and cobalt accelerator when cold-curing the workpiece, never add these two components to the resin and stir them at the same time, otherwise deflagration may occur. Be sure to mix the two substances one after the other!


Storage of Polyester Resin

After you have finished your work, you should close the resin containers tightly and store them in a cool place protected from light. Do not pour over the resin, but store it in the original container. This way it will keep for about six months at room temperature.

MEKP hardeners should not be stored for too long as they may absorb water. This can cause structural damage to top layers during application.



Tips and Tricks

  • Avoid right-angled shapes when working with polyester resin. These are always a weak point. To avoid such shapes, you can use hollow trowels.
  • If you want to make a project with polyester resin weather-resistant, apply a protective coating after the material has cured. This will ensure that even UV light or even the chlorinated water of a pool cannot damage the polyester resin.
  • Make sure that your project is protected from direct UV radiation during the curing process, as this can have a negative effect on its structure.
  • A large surface area of the mixed resin mixture ensures a longer working time, as the polyester resin does not heat up as quickly in this way. If it is your first project with synthetic resin, use a flat but wide tub in which you mix the components or pour the finished mixture into.
  • Surfaces that you want to protect without covering them with foil can also be treated with release agent. When you are finished with your work, simply remove the resin splashes from the treated surfaces.
  • If you want to remove overhanging casting resin, for example if there are noses or casting seams, you can use tools such as scalpels, Dremel, files and various types of saws. However, ensure that there is sufficient ventilation or wear a respirator, as this will also cause the cured material to heat up and thus produce odors.
  • Especially for beginners who want to experiment a little, polyester resin is a good choice as it is comparatively cheap and easier to mix. However, there are several things to consider, especially with regard to safety.

If you follow our tips, you will have a lot of fun working with polyester resin and you will surely create a great project.



Frequently Asked Questions


Can you use Polyester Resin over Epoxy?

Epoxy resin can be used on almost every material and will bond on polyester resin. Polyester resin won’t adhere very well when used over epoxy, so we don’t recommend doing this.