You will need an additional item to perform this activity called Thixotropic or Thixotroping Agent. When added to the silicone base it changes the behaviour of it to a thick butter like material that will cling to overheads and verticals where a standard silicone would simply run away. The disadvantage is the fact that air release is nearly impossible so care must be taken when mixing the additive and secondly when homogenising the catalyst.

Prepare the former as if you were pouring the silicone. At this stage some people choose to paint a thin, unthickened layer of silicone onto the former. This ensures there are no bubbles close to the mould surface. Allow this thin primary layer to cure before continuing.

Next weigh out the required quantity of silicone base and catalyst to suit. Next add the thixotropic to the silicone base and mix using a folding action trying not to entrap air. You will notice the change in consistency. Next add the catalyst and homogenate to a solid colour.

Using a spatula, palette knife, plastic applicator or brush, apply the silicone to the former being careful not to entrap air once more. In the case of a larger former, mix the silicone in stages so as not to run out of potlife during application. Apply silicone to a minimum thickness of 10mm. Make sure that the finish is reasonably smooth and that there are keyways present (for engaging a support case)

Allow to fully cure, 24 hours is advised and then begin work on the support case (refer to later sections about making support cases for your moulds). You can choose either plaster (economical reasons) or fibre glass (for large lightweight cases).

Support cases are essential with a skin mould since the mould will deform readily under very little weight due to the thickness. Allow the case to cure fully and remove carefully.

Finally gently peel away the silicone skin from the former and reassemble in the support case, engaging the keyways to ensure the is no deformation from the original.

Of course the disadvantage of a skin mould is the fact that it will slump with large verticals and will fall away from any overhead. To help avoid this as much as possible use a harder Shore A Silicone which will support itself to a degree and resist slump on the verticals.

Alternate Method

You can also make a skin mould by laying down the former and covering it with an even layer of clay or alginate (alginate will peel away clean from many surfaces where clay may need washing). With clay, a dusting of talc will often help it to release very easily.

Then build your support case as per the later section in either GRP or Plaster. Mark the position of the support case on the base board. Carefully remove this support case and clean of the spacing material from the former.

Replace the support case, make sure you line up your marks, and make two holes in it to allow you to pour in the RTV Silicone and allow the air to escape. Mix the silicone as per normal, de-gass if possible or allow to stand and then pour into the case.

Allow the Silicone to fully cure, overnight if possible, then remove the support case from the silicone followed by the mould from the former. Ensure 95% cure time is reached before you begin any use of the mould

RTV Silicone Skin Mould Guide