Can I thicken silicone and what do I use

Yes, virtually any RTV 2 silicone rubber can be thickened into a spreadable or brushable mass. You do this with a thixotropic additive. It makes the silicone non sagging, so it stays on a vertical wall without running off, while becoming thin again as soon as you move it.

It is important that the additive matches the silicone type. For tin cure condensation silicone use a thixotropic additive intended for that purpose. For platinum cure addition silicone use a suitable additive or fumed silica. The wrong additive can inhibit the cure or change the properties of the rubber.

The order of operations is essential. First mix component A and component B together completely, including the bottom and sides of the mixing cup. Only then add the thixotropic additive, drop by drop or in small portions, and mix vigorously until the mass is homogeneous. Let the mix rest for one to two minutes so the additive can work and air can rise, then stir once more calmly.

The practical test is simple. The silicone is thickened enough when you can apply it about one centimetre thick to a vertical wall with a spatula without it sagging within a minute.

Do not overdose. Too much thixotropic additive makes the silicone short and crumbly, raises the hardness, reduces tear strength and locks air permanently into the mix.

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