How can I avoid bubbles in my casting?

Bubbles experienced while casting urethane resins are the nemesis of casters everywhere. As you may know, urethane resins are moisture sensitive, and often bubbles found in cured urethane plastic are a result of a reaction between the plastic in its liquid state and moisture coming from somewhere.

Common Sources of Moisture that Might React with Liquid Urethane Plastic

• Humidity – urethane plastic will react with moisture in the air, especially if the plastic has a long pot life / working time. Relative humidity for storing and using urethane plastics is 50 % or less.
• Adding Fillers To Urethane Resins – many people will mix fillers (play sand, calcium carbonate, URE-FIL™ fillers, micro-balloons, metal powders, etc.) with urethane resins to achieve different effects, lower the cost of their castings, etc. Many fillers absorb moisture and, when mixed with the moisture sensitive urethane, cause the resin to bubble or even foam. Remedy: spread your filler out on a cookie sheet to a depth of 3/8" (1 cm) and bake in an industrial oven at 150°F/60°C for at least four hours. Let cool before using. This will allow moisture to evaporate.
• Mixing sticks and mixing containers, depending on what they are made of, can introduce moisture. Stirring sticks made of wood can absorb moisture and will transfer that moisture into polyurethane rubber or plastic. Paper buckets will also absorb moisture. Remedy: A humidity-controlled environment (air-conditioned) will help minimise moisture absorption by these elements. Better yet, use only plastic or metal mixing tools and plastic or metal buckets. These do not absorb moisture that could be introduced to your mix.
• Repeated Opening and Closing of Parts A and B – container can introduce moisture from the air to the unused portions, especially on humid days. Remedy: After dispensing place the lids back on the containers as soon as possible and store in a dry cool place. Also, try using Smooth-On's XTEND-IT™, a dry gas blanket designed to extend the shelf life of moisture sensitive polyurethane products by displacing the air in the container.
• Layer Casting – casting in thin sections using a slow-setting liquid plastic (like Smooth-Cast™ 310) gives the plastic time to absorb ambient moisture. Remedy: Cast slow setting resins in a temperature and humidity controlled environment. Or, try using a faster setting casting compound (like Smooth-Cast 300 or 305). This way the material will cure before it has a chance to react with the moisture.
• Over-applying release agent will generate what appear to be champagne bubbles or pinholes in the cured plastic. Always use the "spray-brush-spray" application method to prevent over-application of release agent for best results.