Composite sandwich structures offer several advantages such as reduced weight, high bending/torsion stiffness, superior thermal insulation and excellent acoustic damping over conventional structural materials. One failure mechanism in a composite sandwich structure is the debonding of the composite facesheets from the core structure. A well-formed adhesive fillet at the interface of the honeycomb core cell walls and the laminate is a significant factor in preventing bond failure. Improved adhesion strength with a well-formed adhesive fillet at the interface of the honeycomb core walls and laminate is the aim of sandwich composite manufacturing, and is one area of investigation in this work. Out-of-autoclave (OOA) processing of composites offers several key benefits compared to autoclave processing such as lower manufacturing cost resulting from a lower capital cost and lower energy consumption. In the present work, the effects of moisturized adhesive film, core type, material system and variable levels of vacuum on adhesive fillet formation are evaluated. Aluminum honeycomb and vented aluminum honeycomb are used as core materials in this study. Two material systems of prepregs and adhesive films, Cycom 5320 prepreg with FM 309-1M adhesive, and MTM45-1 prepreg with MTA-241/PK13 adhesive are used to manufacture the panels and evaluate the performance. A quantitative investigation of the adhesive fillet geometry is carried out for all the panels. Manufactured panels are evaluated for peel strength and flatwise tensile strength in accordance with ASTM standards. Various combinations of core material, moisture content, material system and vacuum level are evaluated for effects on adhesive strength and adhesive foaming.