How much thixotropic additive should I add

The dosage depends on the layer you are applying, because each layer of a brush on mould has a different job and therefore a different target viscosity.

For the contact layer, the first thin layer that picks up the detail, you use almost none. Zero to roughly five hundredths of a percent, which comes down to zero to one drop per hundred grams. This layer still needs to flow out, otherwise it traps air and imperfections.

For the build up layer, the thickened layer that gives the mould its thickness, use around half a percent to one percent. At that dosage the silicone stays on a vertical wall and spreads well.

For the finishing layer and for overhead work, for example on a ceiling or the underside of an object, use one to two percent. The silicone then becomes very stiff and stays in place even upside down.

Always add the additive only after components A and B are fully mixed, and dose it drop by drop or in small portions. Measuring and adjusting is better than adding a large amount at once, because you cannot take it back out.

Stay within these ranges. Above two percent the rubber becomes noticeably harder and more brittle, tear strength drops and air remains locked in the mix where it can no longer escape.

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