Fibre-Glass mould making and casting

  1. 1.Using a pair of scissors cut some ‘tissue fibre-glass matting’ and some

‘course chop- strand fibre-glass matting’ into pieces around the size of


  1. 2.Gel-coat is applied directly onto your master’s surface, which has

already been primed with a suitable release agent (PVA blue is a popular release agent for fibre-glass). We leave the gel-coat to cure for around 35 - 45 minutes.

If you are using a Silicone skin Gel-coat is not necessary.

  1. 3.Apply a liberal coating of catalysed GP resin onto the gel-coat or

silicone surface.

Lay a piece of tissue fibre-glass matting over the wet resin surface and using the brush in a stippling action, press the matting into the resin until saturated. Working from the centre out, ensure all the air bubbles are removed from under the matting (Air bubbles are clearly visible through the wetted matting). The next piece goes on overlapping the first, wetted down with more resin on the brush, and matting.

You do NOT need to wait for each layer to set before starting the next.

  1. 4.The next step is to de-laminate the course, chop strand matting we

cut earlier.

Although it can be used as it is, peeling the two layers of fibre-glass apart is helpful because it makes it easier to apply to a complex form, like a human face. Using the same technique of stippling, wet the matting down, again ensure all air bubbles are evacuated from beneath the surface. Usually two layer of this matting is enough for a mould the size of a face. Larger moulds will need more layers to add strength.

  1. 5.This same process is repeated two or three times with a thin tissue-like

fibreglass matting and another two or three times with chopped strand matting. This laminating  of layers of glass-fibre matting and polyester GP resin is what gives the finished product its strength.

  1. 6.In order to avoid glass splinters from the finished piece it is

recommended that a layer of gel-coat is applied to the outside of the fibre-glass mould.

Smother the exterior completely with catalysed gel-coat, ensuring all the splintery textured glass fibre is covered.

7. Once all is fully cured the mould can be removed from the master. If it is a multi-part mould, it can be cracked apart along the shut lines using wedges and small pry bars. This can be tricky sometimes so ask a technician to help you.

Tip: At any stage though out the whole procedure, pigments may be added. Adding pigment when fibre-glassing is helpful to check you are getting an even coverage.

To CAST a fibre-glass object the same method must be used in the same order.


GP Resin

Health and Safety

When working with these resin’s we must always wear protective gloves and goggles, long  but not baggy, sleeves are advisable especially if you have sensitive skin.

The use of an adequately ventilated working area (like our workshop) is essential. If you are using any polyester for a prolonged period or using a particularly large amount, then the additional protection of a respirator, to protect yourself from the fumes is advised. The fumes can make you feel light headed and a little nauseous if you are over exposed to them.

How do I use GP Resin when Fibre-Glassing?

As with most mould making, the most important thing to remember is you must plan ahead, make sure that once cured and solid you can get the object you are moulding out again, this is particularly important if you are making a rigid mould without a skin. Your technician will show you the best way to design your mould.

Creating shut lines or flanges involves making clay, wood and hot glue or pin and tape flange walls. Flange walls are short walls that lay at a right angle to the masters surface. These serve to separate each part of your mould and make it possible to bolt the mould together again once the master has been removed. Sometimes locating keys (dome shaped indentations) need to be added to the flange in order to lock the mould in the correct position once the original is removed.

Care should also be taken when choosing the appropriate course of shut lines and release agents. Always consult a technician before you begin.

Carbon fibre, Kevlar and fibre-glass matting may all be used with gel-coat and the general application technique is the same as when using fibreglass.

‘Fibre-glass’ is a term often used out of context; it is in fact the resin we use with the fibered glass matting that gives it strength, hence its correct name; ‘Glass reinforced plastic’.


Raw GP resin


Multi-part fibre-glass mould-making


GP resin and gel-coat casting


Resin saturated string


GP resin and brass powder


Gel-coat slip casting


Fibre-glass tissue


Chop-strand fibre-glass


    GP resin casting’s


GP resin jewellery castings taken from silicone moulds

How do I use GP Resin for Casting?

GP Resins is a fantastic alternative for more expensive casting resins such as most polyurethane and epoxy resins, in particular, ‘fast cast’ polyurethane resin, which is commonly used in industry as a casting agent.

Casting with polyesters is more difficult than casting with polyurethanes in that, polyesters have a variable catalyst (MEKP), which we may increase or reduce. The percentage we use is dictated by the thickness of the casting to be formed in a single pour.

The minimum recommended catalyst is 0.25% (equivalent to 0.25g catalyst per 100g of resin) and the maximum, which should only be used when casting very small objects or thin sheets, is 5%.

Always check with your technician what percentage is best for you before you mix anything.

We have many pigments which can be easily added and mixed using a variety of techniques I can show you including; marbling, ageing, glowing, and layering effects, to name just a few. As well as using real metals and other filler powders.

Note: To learn more about the various techniques used for casting with GP resin see ‘Cold metal casting and Marbling’

Price is around £4.00 per kilo


Various GP resin casting techniques


GP resin casting’s


GP resin shapes


  GP resin layering
Resin prosthetics