The grain microstructures and hardness of the surface layers of a Mg–RE alloy (3.44 mass % La) and two Mg–Al alloys, (8.77 and 11.6 mass % Al), are compared. All three alloys exhibit a harder surface layer, or skin, with fine primary α (Mg) grains, and a softer core with a larger grain size due to the prevalence of externally solidified grains (ESGs), but the skin of the RE alloy is more continuous and uniform due to the relative absence of ESGs. The distribution of ESGs can be related to the rheology of the liquid–solid mixtures inside the die: the solid fraction in the RE alloy, for given drop in temperature, is much larger than for the Mg–Al alloys. Being more viscous, the liquid in the RE alloy prevents the migration of ESGs towards the surface, hence the more uniform and tightly packed grain microstructure and better defined skin.