If a liquid spreads over a surface, it is said to wet the surface- if it beads up it does not wet. Best example is what happens after you wax a car. Water just beads up on surface and runs off. Any oil film on plastic prevents paints from wetting, but various paints vary in how well they wet. Some people add just the tiniest drop of detergent to water in water-based paint to increase wetting, but a clean surface is the best bet.