Nonuniformities in the coating layer, such as porosity variations and binder distributions, are known to affect print uniformity and barrier properties. However, in the literature the results are rather scarce or sometimes conflicting. We acquired high-quality images of coated paper cross sections using field emission scanning electron microscopy in combination with a new argon-ion-beam milling technique to directly observe and analyze the coating microstructures in relation to underlying base sheet structures. The results showed that coating porosity varied with mass density of the underlying base sheet for the relatively bulky clay/GCC coating, whereas for the more compact clay coating, the effect was small. Areas with more fibers in the base sheet were more compressed by calendering, resulting in a decreased coating porosity. A unique binder enriched layer of less than 500 nm thickness was found at the coating surface as well as at the coating/base sheet interface.