This work aims to improve the adhesive force of epoxy coating with a wood-plastic composite (WPC) surface. First, WPC was formulated using the extrusion technique through different percentages of wood flour (WF) with virgin polyethylene (PE) in the presence of a coupling agent. Dynamic mechanical analysis and water percentage gain (WPG%) were performed to find the optimized WF/PE mixing ratio. The optimized formula was subjected to surface treatment by different methods, including chromic acid oxidation, ultraviolet irradiation, and flame and heat treatment. Then the treated surface of WPC was coated with nano-modified epoxy resin using carboxylated carbon nanotube, organically modified nanoclay (Cloisite-30B), and reactive rubber nanoparticles. The surface treatment of the WPC was characterized with contact angle measurement and Fourier Transform Infrared (FTIR), while the interaction between the WPC and epoxy coat was investigated by attenuated total reflection FTIR microscopy analysis. The adhesion strength of the nano-modified epoxy showed significant enhancement when investigated by pull-off method and cross-cut test. Nano-scratching was utilized in exploring the effect of different surface treatments on enhancing adhesion to epoxy. The interfacial morphological behavior of epoxy coating on the WPC surface was investigated by Scanning Electron Microscope. The results indicated that chemical treatments helped for the functionalization of the WPC surface, leading to stronger adhesion with epoxy. These epoxy-coated WPC composites recorded enhanced mechanical properties because of the chemical reaction between the WPC surface and epoxy.