In this paper, a W-band (75-110 GHz) differential probe system is designed for detecting tight cracks near fasteners and under paint in aircraft fuselage. First, the operation principle of differential probe and the design of the system is introduced, followed by numerical electromagnetic simulations to verify the capability of designed probe. Simulation results show that the designed probe is capable of detecting tight surface crack extending by as much as 1.27 mm beyond a fastener hole, and it is not adversely influenced by the paint coating layer. Finally, the designed probe was mounted on a 3-axis scanner to form an automated scanning crack detection system. The fabricated system was used on many different crack specimens, and the results show that it successfully detected the 2 mm long surface crack (extending beyond a fastener hole) on a lap joint metallic plate with up to 0.1 mm-thick paint. Furthermore, the aperture offset and non-uniform paint clump effects were examined in the simulations, and corroborated in the measurement.