When do I apply the next layer of silicone

Apply the next layer as soon as the previous layer is tacky. Tacky means sticky but no longer liquid. Within that window the layers bond chemically into one body and no visible seam or weak point forms.

The test is simple. Touch the layer with a clean finger on an unimportant spot. The layer may feel clearly sticky, but nothing should come away on your finger. If material transfers, it is too early. If the layer feels dry and smooth, you are too late.

Apply the next layer too early and the layers blend together and your build up slumps, losing detail and giving an uneven wall thickness. Apply it too late and the new layer bonds mechanically but not chemically, and the layers can separate later while the mould is in use.

How long this window lasts depends on the product, the temperature and the humidity. For many condensation silicones it falls somewhere between one and three hours after application. In a cold room it takes longer.

If you are too late and the previous layer has fully cured, do not simply carry on. Abrade the surface lightly and use a silicone adhesive or a primer so the new layer bonds reliably.

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