Here we demonstrate a high performance multifunctional thin primer film made from sandwich structured ultrathin carbon nanotube (CNT) networks and carbon nanotube-epoxy nanocomposite. For this, percolation enhanced CNT-epoxy nanocomposites were fabricated by mixing CNTs with two extremely different length/diameter aspect ratios followed by a unique convective coating process assembling SWNTs into ultrathin, uniform and large-scale film on hydrophobic CNT-epoxy substrates without any pretreatment of the surface. This multifunctional-sandwiched film exhibits relatively low surface roughness, high conductivities, excellent adherence to both polymers and metals, and tunable high EMI-shielding effectiveness of 32.54 dB in the range of 8.2–12.4 GHz (X-band). These results open the pathway to design multifunctional primer thin films that can be used for fabricating hybrid thermal management systems, EMI shielding film and structurally integrated sensors.