This study investigated modifying casein adhesives to improve their moisture resistance and elasticity for use in the wood industry. Six different formulations were tested, including pure casein adhesive, polyvinyl acetate (PVAc) adhesive, and mixtures of the two, as well as modifications involving abietic acid, borax, and diethyl phthalate. While the modified casein adhesives met the minimum requirements according to DIN EN 205, they did not match the performance of the unmodified casein adhesive. Although the modified adhesives performed worse than the unmodified variants, they still demonstrate the potential for sustainable use of non-marketable milk as a source of raw materials.