The fabrication of well-defined, atomically sharp substrate surfaces over a wide range of lattice parameters is reported, which is crucial for atomically regulated epitaxial growth of complex oxide heterostructures. By applying a framework for controlled selective wet etching of complex oxides on the stable rare-earth scandates (REScO3), a(pseudocubic) = 0.394-0.404 nm, the large chemical sensitivity of REScO3 to basic solutions is exploited, which results in reproducible, single-terminated surfaces. Time-of-flight mass-spectroscopy measurements show that after wet etching the surfaces are predominantly ScO2-terminated. Moreover, the morphology study of SrRuO3 thin-film growth gives no evidence for mixed termination. Therefore, it is concluded that the REScO3 surfaces are completely ScO2-terminated.