We discuss the preparation of an atomically flat solid-liquid interface between solid gallium nitride and liquid gallium using in situ surface X-ray diffraction to probe the interface roughness. For the creation of this interface it is necessary to start the experiment with liquid gallium which first etches into the solid at a temperature of 823 K in a nitrogen free ambient. After this rigorous cleaning procedure there is perfect wetting between solid and liquid. The roughness created due to the fast etching of the solid has to be repaired at a nitrogen pressure of 10-20 bar and a temperature around 1150 K. The (2,1) crystal truncation rod data are excellently described by a surface model having 0 +/- 0.1 angstrom roughness, which indicates a successful repair. The lateral length scale on which the roughness is determined has a lower limit of 750 +/- 50 angstrom. (C) 2016 Elsevier B.V. All rights reserved.