Abstract: Water-based primer composition made up of (A) a water-compatibilised modified polyolefin type resin, which is a modified polyolefin resin modified with an .alpha.,.beta.-unsaturated carboxylic acid, of a saponification value of 10-60 mg KOH/g, a number average molecular mass of 4000-30,000 and a degree of chlorination of 0-30 wt %, water-compatibilised by means of an amine compound, and if necessary additionally by a surfactant, (B) a water-compatibilised acrylic resin of a glass transition temperature of -50.degree. to 20.degree. C., and (C) a water-compatibilised polyurethane resin of a break elongation of 200-600%, and if necessary an additional curing agent, in which the contents of the different components are (A) 25-70 wt %, the sum of (B) and (C) 30-75 wt %, and the ratio by weight of (B)/(C) 100/0-50/50.