A paint coated onto polymeric materials does not blister necessarily. This phenomenon is produced by humidity during natural and accelerated ageing and appears in the form of blisters of Various sizes and densities. The substrate studied is polyamide 66. Polyurethane and polyacrylic paints were used. The study involved microanalytical and microstructural techniques (scanning electron microscopy, infrared spectroscopy) as well as macroscopic techniques (water analysis, gravimetry, porosimetry, three-point bend test). Blisters result essentially from organic residues which originate from the substrate and migrate to the paint/substrate interface via an osmotic mechanism as the substrate becomes saturated with water. This study deals with variations of polymer substrate fabrication. Performance improvements can be realized by lowering the amount of mineral fillers in the polymer substrate, by increasing the substrate thickness, and by increasing the mould temperature.