We have studied the prewetting dynamics of H2 on rough Cs substrates obtained by low temperature deposition. The boundary between the thin and the thick van der Waals film is strongly pinned and distorted by the defects of the substrate. Comparing prewetting and wetting dynamics allows us to show that the dynamics and the geometry of the thin-thick boundary cannot be accounted for in a simple 1D model. The finite width of the boundary makes its behavior similar in many aspects to the one of a contact line.