A: This is called "cure inhibition" Cure inhibition occurs when a surface contaminate prevents a material from curing as expected.
Experiencing Inhibition When Making A Mold – When making a mold, cure inhibition occurs when contaminants on a model surface prevent the liquid mold rubber from properly curing. While the cured rubber mold may look fine, you will discover that inhibition occurred at the interface between the rubber and the model’s surface. The rubber will be sticky and not fully cured.
For example, applying latex, urethane or silicone rubber directly over a model made of clay containing sulfur will result in cure inhibition. The sulfur in the clay is preventing the rubber from curing. The rubber may appear fine on the outside, but anywhere the rubber touched the clay, it is sticky and will never cure.
Remedy: Apply a suitable sealer.
Not Allowing A Sealing Agent To Dry or time to “flash off” prior to applying release agent or mixing and applying mold rubber over your model.
Remedy: After applying the required number of coats, let your sealing agent dry thoroughly (30 minutes should be enough) before applying release agent (if necessary) or mold rubber.
Be aware; Humidity will affect the evaporation rate of solvents. The more humid the environment, the longer a sealer or sealer / release combination will take to dry. In more humid environments (80% RH), these solvents can take a couple of days to dry or may never dry depending on the solvent.
Also, if you use a solvent based sealer such as shellac or acrylic and then use a liquid release agent over the top of that, the solvent in the liquid release may "re-liquify" the shellac or acrylic and additional time is necessary to let everything dry.
Applying Rubber Into/Over Rubber - Some mold rubbers are not compatible with others. For example, condensation cure silicones will not cure when applied against an addition cure silicone, and vice-versa.
Minimizing Cure Inhibition – Read the technical data sheet for the products you are using. It should tell you under what circumstances inhibition would occur and what you can do to prevent it. For example, applying a sealing agent to a sulfur clay model before applying mold rubber will prevent cure inhibition in most cases.