Thin films of mercury based superconductors were prepared on the R-plane sapphire with a CeO2 buffer layer using a two step process involving the deposition of the Hg-free precursor and ex situ mercuration in the sealed quartz tube. For the thin film preparation, a method with no contact between the mercury source and the precursor film was used for mercuration. We studied the influence of the basic parameters of the mercuration (annealing time, temperature and partial pressure of the mercury) regarding the composition and superconducting properties of prepared Hg-based films to determine appropriate conditions for this mercuration method. We found out that an increased partial pressure of mercury inhibited the creation of the parasitic Re-based phase and supported the crystallization of the superconducting phase. The advantage of this mercuration method is higher reproducibility as well as its capability to prepare high quality thin large areas films.