A novel waterborne coating has been successfully designed by combining good barrier properties and a built-in ability to in situ phosphatize low carbon steel substrates. The physical barrier protection was promoted by a homogeneous dispersion of crystalline nanodomains in the polymer matrix, which was achieved by the coalescence of polymer particles with core-shell morphology. The core consisted of semicrystalline polystearyl acrylate, PSA, and the shell consisted of a film forming MMA/BA copolymer. The in situ phosphatization was provided by the incorporation of phosphate functionalities on the particle surface that were able to interact with the steel substrate during film formation. This combined functionality yielded films with excellent corrosion resistance properties as measured by electrochemical impedance spectroscopy analysis of samples either immersed in 3.5 wt % NaCl solution or subjected to an aggressive salt spray chamber. Thus, films containing 40 wt % of PSA and 2 wt % of phosphate surfmer provided corrosion resistance over more than 800 h when exposed to harsh salt-spray chamber conditions.