Traditional vehicles and aircraft have consisted primarily of steel and aluminum alloys which due to their density, has resulted in various logistic problems including transportation, maneuverability, fuel efficiency. These hindrances have led to a major increase in the incorporation of composites material into this equipment. As the use of these composite increases, multi-substrate coating systems that can provide adequate corrosion protection to metal components, as well as superior adhesion and flexibility to the composites, are needed. The goal of this work is the modification of diglycidyl ether of bisphenol-A epoxy coating systems for improved flexibility and adhesion for the development of multi-substrate primer. For this purpose, commercial additives of various chemistry (liquid rubber, polysulfide, novolac phenolic resins, silane coupling agents, and polyethers) were incorporated into a model epoxy-polyamine coating and their effect on flexibility and adhesion was investigated. Based on their performance, the top-performing additives were incorporated into a fully pigmented modified primer. Overall, the studies in this dissertation not only demonstrated improved flexibility and adhesion to metal and composites but also improved overall corrosion protection compared to an unmodified primer.