Hi, I would wash the model with a 50-50 white vinegar and water solution to get rid of any oils, an old toothbrush will help shift any debris in the detailing. Allow to dry and, handling by the edges to avoid any skin oils getting on the visible surface, spray a light mist of ordinary grey plastic primer over the model - just to give your chosen paint something to key to whilst minimising loss of detail.   You could strip off the original paint, but I've never attempted that on a resin model