Why does my silicone not cure against the original

When the top surface of the silicone cures nicely but the side that sat against the original stays tacky or liquid, you are dealing with inhibition. The substrate is locally blocking the chemical reaction.

This occurs almost exclusively with platinum cure addition silicones. The platinum catalyst is extremely sensitive to sulphur, tin, latex, styrene, certain adhesives, certain modelling clays and residues of particular lacquers. Polyester is a well known example, because it keeps releasing styrene vapour long after manufacture. That vapour penetrates even a layer of release wax.

There are two solutions. Seal the original completely with an acrylic or polyurethane lacquer, let it cure fully and only then apply the release agent. Or switch to tin cure condensation silicone, which is insensitive to these substances.

Always run a drop test before making the full mould. Apply a small amount of mixed silicone to an inconspicuous spot on the original and let it cure completely. If it stays tacky, the substrate is unsuitable or not properly sealed.

With condensation silicone the cause of an uncured layer usually lies elsewhere, namely in an inaccurate mix ratio, in incomplete mixing along the bottom and sides of the mixing cup, or in a very dry environment.

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