Improvement of the paint adhesion to a polypropylene (PP) bumper has been investigated without using a primer by treating the bumper surface with O-2, H2O, and acetylene plasmas. All the plasma treatments resulted in an increase of the adhesion strength in dry conditions. The adhesion strength could be increased up to a value comparable to that obtained by applying a primer. The: treated surfaces were quite stable for 7 days in air. After exposure to wet conditions, however, the adhesion strengths for both O-2 and H2O plasma-treated samples decreased significantly, while the adhesion strength for the acetylene plasma-treated sample did nor change much.