Preparing Silicon Inner For Fiberglass Moulds

HI! Just another up date about what we have been getting up to in the special effects studio. Last week we were making a start on a different type of mould –  Fiberglass outer and silicone inner.

First we had to make the silicone part of the mould. To do this we used life casts of fellow class mates we had made last semester. If you would like to have a quick look of how we did that just click the following link:

So here goes!

You will need:
– Clay
– Mod-Roc
– Vaseline
– x2 cardboard cups
– Silicone part A and B / Accelerator (optional)
– Measuring scales
– Wooden sticks (lolly pop sticks)
– Gloves Optional)
-Plastic bowl


First began by applying a light layer Vaseline to the already shellaced face cast. This ensured the silicone we were about to apply would not adhere to the surface of the plaster. Next we placed the positive onto a wooden board to be able to build up a clay and Mod-Roc wall around the positive. This ensures the silicone would not spill out over the table. To make it easier to make a neat clay wall we used a clay block to flatten out the clay so we were then able to cut it evenly to fit around the plaster. As an extra precaution I blended the clay onto the wooden board. We then added a few layers of Mod-Roc around the clay.


Once prepped, we were then able to begin mixing the silicone.  First we made sure we had all the necessary materials and tools to do this.

KathrenElizabethMake-upFaceCasting (41)

We first began by zeroing the scales whilst the plastic bowl was on top to make sure we get an accurate reading as it is vital that the silicone is measured out precisely as it will not vulcanize.  The equation for silicone as are follows:

Part A of silicone:

Part B Silicone:


For the size of the positive face cast I was covering I wanted to mix up 100 grams of part -A 10grams part -B and 1gram of accelerator to speed up the vulcanisation process.

KathrenElizabethMake-upFaceCasting (35) Before mixing we made sure that the tubs of silicone were very close to the scales as it can drip really easily which can be difficult to clean up when not mixed as it stays in its liquid form. To avoid and drips whilst measuring out the silicone we also used a wooden stick to scrape the excess silicone from the sides of the cups. We first measured out parts A and B and then added the accelerant and mixed them together really well to ensure all the silicone would cure.

Tip! An important thing to remember when mixing up a batch of silicone is to avoid cross contamination of each part as this could ruin the full tub of silicone and its isn’t cheap! To avoid this make sure to use different cups and wooden sticks when measuring out each part.


We then poured the silicone into the centre of the positive to allow it to drip over the entire surface. This is only the first layer so it does not need to fill to the top of the clay wall.

Tip! To avoid air bubbles during this process it is always best to pour the silicone from a height as this eliminates the hair bubbles as it pours.


We then set the positive aside on a flat surface to allow the silicone to level out and cure. Due to the accelerator we would be able to come back and add the next layer in around 2 hours.


The next later of silicone was of a different consistency to the first as we used a different product – Thixo Additive. We carried out the same process as the first layer however we mixed the Thixo at the very end. There was no precise measurement for this ingredient as it was simply to thicken the silicone up to a state I was happy with. The consistency I was looking for thick paste which I would be able to apply generously over the first layer of silicone. If the first layer is still a little tacky that does not matter as it will help the next layer adhere better.

DSCF0118Once the past was mixed we then used another wooden tool to spread the mixture around (Some may fine using a brush easier – but keep in keep in mind it will damage the brush – so think cheap and cheerful!).


We then left it to cure for a further 2-3 hours.

 DSCF0112-horz DSCF0112

Once the second later had cured, we then added another liquid layer over the top (same process as the first layer) this smooths over any lumps and bumps from the second coat.

Then that is it for the silicone inner! I’ll be posting  the next step very soon!

Thanks for reading 

Katy x


One thought on “Preparing Silicon Inner For Fiberglass Moulds

  1. Pingback: Making the Fiberglass Positive | Kathren Elizabeth

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