The present invention relates to a process for preparing an aqueous dispersion of neutralized multistage polymer particles comprising the steps of contacting under emulsion polymerization conditions a) an aqueous dispersion of first polymer particles with b) first monomers to form an aqueous dispersion of 2-stage polymer particles; then contacting under emulsion polymerization conditions the aqueous dispersion of 2-stage polymer particles with second monomers comprising to form an unneutralized aqueous dispersion of 3-stage polymer particles; then neutralizing the unneutralized aqueous dispersion of 3-stage polymer particles with a base to form an aqueous dispersion of neutralized 3-stage polymer particles, wherein the first and second monomers form a shell having a calculated Tg of less than 50° C. The process of the present invention is useful in preparing a composition that is useful as an open time additive in coatings formulations.