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Cellulose nanofibrils (CNFs) have been studied as a potential component in packaging applications and as standalone films because of their remarkable barrier properties toward grease, oil, and oxygen. Fiber type, pretreatments before fibrillation, fibrillation methods, and other parameters can influence the CNF quality. CNFs produced using a refiner may be coarser than CNFs manufactured with pretreatments such as oxidation with the 2,2,6,6-tetramethylpiperidine-1-oxyl radical, but refiner CNFs can offer an economical option that may still have desired properties. For refiner CNFs, the forming parameters and basis weight influence on the final structure and barrier properties is not well understood. CNF films at different basis weights were formed by film casting, and vacuum filtration methods were followed by restrained drying. As the basis weight increases, the density of the film increased for both formation methods. Cast films had higher densities than filtration-formed films. All samples had mechanical properties that scaled with the basis weight of the film. Crosssectional images determined that the internal structure of these films showed a layered arrangement separated by voids. A dense surface skin layer formed on the films which may contribute to the excellent grease barrier properties even after folding. The layered core structure with elongated voids may dissipate energy during folding allowing deformation to occur without damage to the surface layers, which likely contribute to the water vapor and grease barrier properties that do not deteriorate with folding. The development of the skin-core structure and density were explored by freezing the samples at different times during the drying process, freezedrying, and imaging with a scanning electron microscope to determine the critical solids contents to form a continuous surface skin. Surface skins formed early in the drying process and generated grease barrier properties early during drying. A mechanism is proposed for the evolution of the CNF film structure throughout the drying process, which is similar to the skin formation during the drying of polymeric solutions.