Air bubbles present as round or spherical metal defects or foam-like surface roughness. This defect indicates that the metal managed to penetrate the primary coat; that air is trapped against the wax pattern and the shell formed around it; or there was a void from an air bubble at the surface of the wax. 

The causes of this defect are evident in the shell portion of the process. To cure these causes, R&R recommends taking the following actions.