A method of treating the bristles of a tuft of bristles of the brush, nail varnish application assembly requiring such a brush, and a brush for the application of a make-up or beauty product, having a tuft (2) of approximately parallel bristles (3), a first end of these bristles (3) being fastened to a free end (4a) of a wand (4), the bristles (3) of the tuft (2) being surface-impregnated with a partially acetylated polyvinyl alcohol comprising at least units of formulae (I) and (II): ##STR1## in which the units of formulae (II) are present in proportions ranging from 3 mol % to 40 mol % with respect to the polymer, the polyvinyl alcohol having a weight-average molecular weight ranging from 10,000 to 190,000.