Our study focuses on the use of a remote atmospheric pressure plasma process for the improvement of polymer surface reactivity. Indeed, surface activation of carbon-fiber-reinforced polymers that use a polyetheretherketone matrix is required to achieve strong and long-term adhesion of paint on the composite. Emphasis will be placed on the contribution of gas plasma used on surface modification. We use two techniques to characterize adherence improvement: a qualitative and industrial approach with a cross-cut test and a quantitative approach with a three-point bending test. However, to provide a better understanding of the plasma-induced modifications using air or nitrogen gas, diagnostics of the surface are also necessary. The wettability improvement, chemical surface modifications, and topography are analyzed by using contact angle measurement, X-ray photoelectron spectroscopy, and atomic force microscopy, respectively. The combination of these diagnostics highlights the capability of plasma treatment to tune the plasma/surface interactions depending on gas plasma, which results in improved adhesion.