For a surface to be created, the cohesive bonds between the molecules in a substrate must be disrupted. This results in a surface energy or tension between these molecules. What is happening, is that the molecules on the surface are attracted to like molecules, and want to bond with like molecules on all sides (like the molecules under the surface).

This can be a major factor how much material it will take to cover a surface. If a substrate has a high surface energy, it has a strong desire to bond to something. A coating will be pulled down to the surface and “wet out”, causing it to thin. This also helps draw a coating in and increase the number of “contact points” with the surface, which will improve adhesion. If a substrate has a low surface energy, the coating will be less attracted to the surface. As a result the coating will have more of a tendency to bead up (It may not be as extreme as oil & water but it does happen). This will make the coating go on thicker and as a result, not go as far.