The present invention relates to a composition comprising at least one masked or unmasked polyisocyanate in the form of an aqueous emulsion whose mean size is between 0.1 and 10 micometers; and at least one polyhydroxylated polymer, advantageously a polyol, in the form of a second aqueous dispersion whose mean size is between 10 and 200 nanometers wherein the content of polyhydroxylated polymer, advantageously of polyol, expressed in mass per cent, is at least equal to 5/6R-20, with R equal to the ratio of the mean size (d₅₀) of the emulsion to that of the dispersion of polyhydroxylated polymer, advantageously of polyol.