Affix the model to a smooth clean dry surface. Carefully construct a tight case around the model.  Make sure that the model and case have been properly prepped and sprayed using the correct releasing agent.  Polyurethane RTV Rubber will stick tenaciously to anything that has not been properly sprayed with release.

The Polyurethane RTV Rubber comes in two parts.  Weigh each part correctly and accurately.  Mix by hand or mechanically means for 3 to 5 minutes or until thoroughly mixed in a super clean pail preferably plastic. After pouring the Polyurethane Rubber, allow the mixture to cure for 24 to 30 hours at room temperature before de-molding.

When making a rubber mold avoid contaminating the mixture with bits of clay and plaster chips, droplets of water, wood chips, etc.

The Polyurethane RTV Rubber molds are usually locked into a Hydro Stone or Fiberglass 'mother mold' which acts as a of base of support for the rubber.  Fiberglass bases may crack  over time.  The 'mother mold'  base will stabilize the rubber mold during casting, otherwise, distortion in the final product could occur.  This is particularly true when casting plaster, concrete, wax or soap items.

When not is use, store the rubber molds carefully on a flat surface to avoid warping and twisting in a cool, dry area and avoid directly contact with UV rays (direct sunlight). Silicone Molds are still by far the best mold material to use for all types of applications.  Polyurethane may be less expensive when first purchased, but remember, you get what you pay for.