How to cast with gelcoat & fibreglass

Learn how to cast with gelcoat & fibreglass from a silicone mold


If you have an old silicone mold laying around and are looking to revive and create a few replicas, or maybe you have a new mold with complex shapes or parts, then casting with gelcoat and fibreglass is quite the viable option.

Gelcoat and fibreglass are both very flexible and durable materials that will allow you to reproduce many copies of your pieces that will last for years to come. Not only are these materials used in the creation of some of your trusted cars and boats, they are well suited to create amazing sculptural pieces.

In this article you’ll get a breakdown of how to apply your gelcoat, fibreglass matting and polyester resin to an existing silicone mold, housed in a fibreglass mother mold. The mold depicted will be that of a fish, however the same process can be applied to other molds with varying designs.

The materials you will need are your:

Existing mold

Fibreglass matting

Gelcoat 

Polyester resin

Mold release spray

Talcum powder

Metal painting tray

Paint brush

Mounting wood block (optional)

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As a precaution, when working with any material that gives off harmful fumes, preparation and safety is important. Always ensure that your environment is properly ventilated, wear protective gears like a respirator and rubber gloves. 

Now that you are prepared let’s get into it.

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Step 1:  Preparing your materials

Before you get started with adding your gelcoat, make sure that you prepare your sheets of fibreglass by tearing them into relatively small pieces. 

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You also want to prepare the mold by spraying mold release on the fibreglass mother mold, between the silicone mold and the fibreglass mother mold. This will ensure that the fibreglass that you’ll be adding to do the casting, does not stick to the mother mold.


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Adding talcum powder to the silicone mold is also recommended as the powder acts as a mold release when the fibreglass is added to the mold and gelcoat. The powder also makes it a bit easier for painting when the cast is finished.

To do this add a fair amount of talcum powder to the mold, and with a dry brush evenly distribute the powder throughout the mold. Then shake and remove the excess loose powder.

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Step 2:  Mixing the gelcoat

The gelcoat is a resin that is often used as the top coat or first layer of resin that is applied to the surface of a mold. This is used to create a hard smooth finish while protecting the fibreglass. To get started, mix your gelcoat with your MEKP hardener, the measurement for this particular mold depicted is 100 millimeters with 1% MEKP catalyst. You then want to mix the combination for about 60 seconds.

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Step 3:  Adding the gelcoat

Once your mixture is complete, it’s time to add it to your mold. Holding the mixture a few inches above the mold, you want to gently pour it to get a stream of gelcoat into the mold. This method will help to eliminate any unwanted air in the gelcoat.

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Try to ensure that the gelcoat gets into all the intricate areas, like the fins of the fish on the mold. 

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Once you’ve done that, using a popsicle stick or a piece of wood you want to add the mixture around the inner edges of the mold. 

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Using a dry paintbrush, push, not brush the gelcoat into the areas of the mold allowing gravity to help move the gelcoat into the recesses of the mold. The gelcoat has a honey like consistency so you do want to ensure that it is getting into all the nooks and crannies. The aim is to ensure that the first coat is perfect and there is no air trapped in it, you can also go around the mold twice to make sure that the areas with the most complex design like the fins, are properly coated. 

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Once the mold is properly coated, remember to clean the brush off with some acetone and allow the gelcoat to sit for about 60 minutes. The gelcoat should have a tacky consistency, but not too much where the gelcoat comes off on your finger.

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Step 4:  Layering the fibreglass matting and polyester resin

Next it’s time to add your fibreglass. Take up a few sheets of your fibreglass matting and begin to trim off a few pieces to create loose fibreglass strands all over the first gel coat.

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Once the surface is covered with the loose strands of fibreglass, now it’s time to add your polyester resin.

Grab your mixture of polyester resin and MEKP catalyst, which is the same as the one used in the gelcoat. The measurement for this mixture is 300 millimeters of fibreglass mixed in with 1% to 2% MEKP catalyst, thoroughly mixed for about 60 seconds.

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You want to then pour the mixture into a metal tray, if you have a metal paint tray that is perfect, you can add it to the well section of the paint tray. 

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Using your paint brush apply a neat layer of polyester resin on top of the gelcoat and fibreglass, ensuring that the glass fibres are well soaked with the resin.

From the strips of fibreglass that you prepared at the beginning, you can move on to now adding more layers of fibreglass with resin. To do this dip a few of the strips into the polyester resin only on one side and place them on the painting tray. Use your brush to further dap some of the resin on top of the fibreglass sheets.

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When all the sheets have been saturated with resin, the first fibreglass strip is ready to be applied to the mold on top of the gel coat that was just applied. Some of the matting may have straight edges so make sure that you align those with straight edges with the straight areas of the mold.

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Continue to evenly layer and overlap the desired amount of fibreglass matting, while dabbing on an even spread of resin on top of the fibreglass as you move through the mold. For smaller areas like the fins, apply a smaller strip of fibreglass matting.

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If you intend on mounting your piece, you can prepare a piece of wood that would fit into the center of the mold and act as a mounting block. This wood will also be casted in fibreglass and resin. To do this dip the wood in the resin mixture and rest it in the center of the mold, then add a small strip of fibreglass with resin underneath to level that block out, and some more fibreglass on the block to secure it in place.

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Before adding your final layer of fibreglass, allow the layer you just completed to cure for about an hour, or until it is hard enough and the block is firmly in place.

While your piece is curing, remember to clean your tray and brushes to prepare for your final layer.

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After 60 minutes, using the same method of applying the fibreglass and polyester resin, add your last layers until you have a nice solid cast. 

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Step 5:  Decasting the mold

Once the casting has been thoroughly dried, it’s time to remove it from the mold. If done correctly you should have a perfect replica of your design and in this instance a fish. Before removing your casting, remove the fibreglass mother mold and then proceed to gently peel back the silicone mold from the cast. 

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After you’ve removed the piece you may notice that you have a few ends that may need some cleaning up, you can simply use a dremel or whatever tool you may have to clean that up, and you’re now set for painting.

So there you have it folks, you too can create your own crazy, creative pieces with gelcoat and fibreglass. It’s flexible, durable and lots of messy fun.

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