Bubbles typically mean that air has been trapped in the mixture. The product should be mixed slowly and poured in one area, the lowest part of the mold and allowed to naturally spread into the mold. A pressure pot can be used to remove bubbles before pouring, but the most foolproof way to avoid bubbles is to pressure cast or vacuum cast after the rubber has been poured into the mold. Sealing the mold and using the correct mold release will also help avoid surface bubbles.