Making a fibreglass positive

Hi everyone this post will be all about the next step of fibreglass mould making. So for this post i will be taking the silicone off my plaster models and filling this with fibre glass to get a positive fiberglass mould of my models face which i will then go to sculpt my prosthetic onto . So here we go :

Products neede:

  • Fiber glass matting
  • Gel coat
  • Fiberglass matting
  • Plastic sheeting
  • Scissors
  • laminating fibreglass resin
  • paint brush
  • Tools to use to relive the silicone from the plaster mould


  • The first step is to realise the silicone and fiberglass topping from the plaster life cast .                                                                                     1 (2)1 (4)
  • Use a  wooden sculpting tool and  gently prized the two sections apart (be careful not to damage any part of the mould as this can affect the final product .  This can be time consuming and difficult as the two can suction together .
  • place the plaster live casts to one side to create more space for the next step.
  •  cover the work surface  with plastic as fiberglass will adhere to anything and not come off .
  • You need to fix the negative to a wooden board to make sure it does not move when applying the fibre glass to the inside. To do this you need to make tow sausage shapes of clay to the front and back of the negative .                                                                           1 (6)1 (10)
  • mixed up a half cup of gel coat to begin the fibreglass process. Same as before you need to  use the same ratio of catalyst and applied I layer all over the silicone inner. This will pick up the detail of the silicone giving us a perfect surface to sculpt on top of. Next  added a generous layer of fiberglass fibres on  top of the gel coat this adds extra strength to the mould .                              1 (19)

IMPORTANT INFORMATION : Never place the cups and brushes directly into the bin after use – Fibre glass heats up when it cures, meaning it is a fire hazard. It is best to let it cure in sight completely.

  • Before the gel coat dries  you can add colouring  to the gel coat to be a bit more creative . (For my mould i added a brass colouring so it would give my mould a metallic colouring when polished up ) .                                                                                                    1 (14)1 (18)
  • Once the gel coat is  applied  then leave  it to cure for around an hour. This will  allow the gel coat to cure but not become completely dry – leaving the surface slightly tacky ready for the next step.
  •  Cut 15-17pieces of  3inch square pieces of fibreglass matting ready for the next step .                                                                                     DSCF0183
  • Lay out a plastic sheet to protect the work area from fiberglass.                                                                                                                                 DSCF0184
  • Measure out half a cup of laminating fibreglass resin make sure to use a new cup to prevent cross contamination of products. Next add the catalyst to the resin using the same measurement as 2 from the last step and mix it thoroughly.                                      DSCF0175DSCF0179DSCF0180
  • Use a brush to paste a layer over the tacky surface of the gel coat this will help the next layer adhere. Then placed four pieces of the matting onto the plastic sheeting and soak  them with the resin.                                                                                                                   DSCF0185
  •  Once all four have been coated take the first square and place it into the cast over the surface of the painted layer of the resin. Past another coat over the top of the matting to make sure it is flat without any air bubbles. Also make sure each square overlapped slightly to make sure there is no  gaps where it could make the cast weaker .                                                                               DSCF0187DSCF0188DSCF0031
  • Leave it to it to dry for around 20 minutes until it goes a green colour you can then use a pair of scissors to cut off a excess fibre glass hanging off the mould edge .  If you do not do this and leave it too long the fiberglass will harden too much to cut and you will ned to use a grinder power tool to take off the sharp edges .                                                                                                                 DSCF0034
  • repeated the process  and add a second layer Then leave it to dry completely overnight.
  • Once dry  you can separate the silicone from the fibre glass using metal sculpting tools (Be patient and carful , you don’t want to damage your mould ) Once the two parts were separated we were left with the fiberglass positive! I was shocked to see how much detail the gel coat had picked up.                                                                                                                                                                                 DSCF0040
  • As i added a brass colouring to the gel coat i then buffed up the surface of  the cast using wire woo so i could see the metallic shine .                                                                                                                                                                                                                                                     DSCF0045


At the beginning of this mould making process I didn’t really understand why we created the silicone part of the mould , but by doing this next step I now understand this process . I am a very visual learner and i need to do a process to fully understand it.  It also gave me a light weight positive to sculpt on , this will be  much easier than sculpting onto a heavy plaster positive . I am really pleased at how my positive has turned out and it has made me realise that you have to be very precise and carful with every step to achieve a good final mould.

Thank you for reading



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