The invention relates to an aqueous dispersion for use as open time improver in a coating composition which aqueous polymer dispersion comprises a first polymer having a number average molecular weight (Mn) of from 2,000 to 120,000 (determined by gel permeation chromatography using a mixture of tetrahydrofurane and acetic acid as eluent), an acid value of from 30 to 150 mg KOH/g, and an ethylene-oxide wt % (on total solid polymer) of from 1 to 20 wt %, said first polymer dispersion being obtainable by free radical polymerization of a monomer mixture in the presence of at least one free-radical initiator and at least one surfactant, said monomer mixture comprising: a) 5 to 20 wt %, preferably 7 to 10 wt %, acid functional ethylenically unsaturated monomers or precursors thereof or ethylenically unsaturated monomers comprising ionic group precursors; b) 5 to 25 wt %, preferably 7 to 20 wt %, ethylenically unsaturated monomers containing polyethylene oxide, polyethylene glycol or mono-alkoxypolyethylene glycol moeity c) up to 90 wt % of non-ionic ethylenically unsaturated monomers other than a) or b); d) 0 to 10 wt % ethylenically unsaturated monomers with a functional group for cross-linking e) 0 to 10 wt % of chain transfer agents; f) up to 90 wt % non-ionic ethylenically unsaturated monomers other than c), wherein 30 to 90 wt %, more preferably 60 to 80 wt % comprise crosslinkable groups or precursors thereof; wherein the sum of a) through f) is 100 wt %. The invention further relates to a method for making the first polymer dispersion, the use of said aqueous dispersion as an open time improver in a coating composition, to aqueous coating compositions comprising a blend of at least a first aqueous polymer dispersion and a second aqueous polymer dispersion comprising a film-forming second polymer and to a method for making said coating composition