Graphene due to its two-dimensional structure, large surface area and high impermeability is regarded as an excellent functional filler for the development of anti-corrosive coatings by creating a natural barrier to the diffusion of electrolytes. Epoxy polymers are widely used as protective coatings, and in the present study, commercially-available graphene nanoplatelets (GNPs) were dispersed into an epoxy resin using three-roll milling (3RM). The GNP-modified epoxy was coated onto mild steel substrates, and cured. The coated panels were immersed into a corrosive environment of 3.5 wt% NaCl aqueous solution for 4–5 days. The adhesion of the coatings to the substrate was then measured using a cross-cut test. The addition of higher loadings of GNPs resulted in a deteriorating corrosion performance, with the 1.5 wt% and 3 wt% coatings exhibiting 53% and 91% damage, respectively, after the cross-cut tests. The unmodified epoxy and low GNP content coatings (≤0.5 wt%) demonstrated 0% damage. This shows that the corrosion behaviour of GNP/epoxy coatings is not dominated by barrier effects but by electrochemical factors. The addition of GNPs is only effective at low loadings, as higher contents result in electrically-conductive coatings that facilitate the conduction of corrosion currents.