Bubbles usually indicate that there is trapped air in the mix. One reason bubbles form on the rubber’s surface is due to moisture. Urethane is moisture sensitive and may have reacted with moisture within the mold box or environment. Moisture can come from humidity, a wet or porous model, or wet mixing containers and sticks.

Another reason air bubbles may have formed could be from using too much mold release. While it’s important to use release to ensure the material will demold from the molding surface, using too much will affect the cured surface as well.