Refill friction stir spot welding (RFSSW) is a derivative of friction stir spot welding (FSSW). Unlike conventional FSSW, it does not leave behind an exit hole in the workpiece. Kawasaki Heavy Industries (KHI) developed a novel robotic RFSSW system for aerospace applications. To date, researchers documented numerous studies on refill friction stir spot welding of bare aerospace aluminum alloys. This study explores further and demonstrates refill friction stir spot welding of surface-treated dissimilar aluminum alloys with a faying-surface sealant. Surface treatments are vital for corrosion inhibition and paint adhesion of aircraft structures. An ability to join metals with surface treatments is an enabler in implementing the RFSSW technology in aerospace applications. The presented study features the welding process development, mechanical tests, and comparisons between bare and surface-treated RFSSW coupons.