Sand-cast compacted graphite (CG) cast iron typically presents a surface layer (the casting skin) whose microstructure is significantly different than that of the bulk material. It is generally believed that the casting skin is the result on interaction between the metal and the mold moisture and atmosphere. This reaction results in a decarburized or graphite-free layer. The thickness of this layer is a function of process variables (e.g. cooling rate, pouring temperature, binder, coating, sand fineness, and Mg and inoculation level). The paper presents a summary of recent findings on the negative effect of casting skin on the static properties of CG iron through the use of the skin quality factor for tensile strength, defined as the ratio between the as-cast and machined tensile strength. A mechanism of casting skin formation is proposed and supported with experimental and computational data.