Prerequisite for the adhesion of a glue or a coat on a surface is the wettability. To make wetting possible, the surface energy of the substrate must be greater than that of the coating. In particular a great many plastics have a very low surface energy, making bonding difficult. Often, a plasma treatment is intended to make a surface wettable and thus gluable. Wettability can be easily tested with test inks. Diener electronic offers a test ink kit which is suitable for most practical applications. It comprises 7 liquids with surface tensions between 28 and 105 mN/m. Additionally, further settings between 18 and 105 mN/M are available. If a test ink dissolves on the substrate, the surface tension of the substrate is greater than the surface tension of the ink. The quality of the test inks and test procedures is in compliance with the standards DIN 53364 and ISO 8296.