How do I prevent air bubbles in a brush on mould

Air bubbles in a brush on mould almost always come from the way the silicone is applied and not from the material itself. First establish where the bubbles are. Bubbles within the wall thickness are cosmetic and have no effect on the castings. Only bubbles in the mould surface, meaning the side that sat against the original, will reappear in every product.

Apply the first layer wafer thin, around half a millimetre, and leave this layer virtually unthickened. Thickened silicone can no longer degas itself, so any air inside it stays inside. Stipple the silicone into the surface with the tip of the brush and do not brush back and forth, because every return stroke folds air under the layer.

Finish internal corners and fine detail first. Air is most persistent there because the brush never truly reaches the corner and the silicone bridges over it. Push the silicone into the corner with a dabbing motion before working on.

Mix calmly with a scraping action rather than whipping or at high speed, and let the mix stand for one to two minutes after thickening so larger bubbles can rise. Pop visible bubbles immediately with a needle and smooth the spot, because once the silicone starts to grab this is no longer possible.

Work at eighteen to twenty five degrees. Cold silicone is thicker and holds on to air. Seal porous substrates such as plaster, concrete and wood first, because these materials release air as soon as the silicone sits on them.

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