Potential links between pressure conditions during co-cure of honeycomb sandwich panels, the extent of gas flow through facesheet and bond-line, and the level of permeability in the cured skin were evaluated. Half-sandwich structures comprised of fiber-reinforced polymer facesheets, film adhesive, and core were fabricated using a custom-built lab fixture. Autoclave, bag, and core pressures were varied to produce controlled, constant pressure differences during cure, and the resulting skins were tested for permeability using a fixture constructed to measure gas flow rate across the skins and to locate gas flow pathways. Facesheet cross-sections were analyzed to evaluate porosity. Porosity and the number of gas flow pathways were correlated to permeability, but significant gas flow was possible without high void content or with few channels, as pressure differentials led to complex variations in permeability. Overall, the study provides new insights into gas transport during composites processing and manufacturing, and the results provide guidance for modifying manufacturing processes to ensure part quality.