How do I apply silicone to a vertical wall without it running off

On a vertical wall liquid silicone sags under its own weight. Thin or open spots appear at the top and material accumulates at the bottom. That is exactly where air bubbles between the layers appear later.

The first thin contact layer must not be thickened, because then it no longer captures detail. Apply that layer in two passes instead. First a wafer thin layer, wait three to five minutes so the silicone begins to grab, then apply a second thin layer. This way the layer stays put without any thickening.

For the following layers, thicken the silicone with a thixotropic additive, around half a percent to one percent. The silicone will then stay on a vertical wall. Build up in thin passes of two to three millimetres and work from the bottom upwards, always in the same direction. Material that sags then lands on an already filled zone instead of an empty one.

Press the silicone on with the spatula instead of dropping blobs. A blob wiped off the spatula almost always carries air with it. Place the spatula flat and draw it out.

Do not work too cold. Below eighteen degrees the silicone is thicker but also bonds less well to the previous layer, and above twenty five degrees the working time shortens considerably.

Read more: complete guide to making a brush on silicone mould