Which substrates should I seal before applying silicone

There are two reasons to seal an original. The first is porosity, where the silicone keys mechanically into the surface or where the material releases air and moisture. The second is chemical activity, where the substrate blocks the cure of the silicone.

Porous substrates that must always be sealed are plaster, concrete, natural stone, brick, untreated wood and MDF. Use a sealer, shellac or primer here, let it cure completely and only then apply a release agent.

Chemically active substrates need extra attention when you work with platinum cure addition silicone. Polyester and gelcoat release styrene vapour, plasticine and sulphur based clay release sulphur, latex and rubber contain sulphur and plasticisers, and soft PVC contains plasticisers that migrate. Seal these materials with an acrylic or polyurethane lacquer, or choose tin cure condensation silicone, which is insensitive to these substances.

A 3D print in PLA or resin must be fully post cured before you mould it, because residual monomer can inhibit the cure. Sand and prime the print as well to remove layer lines.

Metal, glass and glazed ceramic do not need sealing, but do need degreasing and a release agent, because silicone bonds well to smooth surfaces.

When in doubt, always run a drop test. Apply a small amount of mixed silicone to an inconspicuous spot and let it cure completely. If it stays tacky, the preparation is not sufficient.

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