Draw back of dark grounds

We had to paint copies off heads from a master painting for Terry Miura's class. I realized that Madame X by Sargent didnt work on the dark grounds. It is impossible to get complete coverage of the dark values on the slippery oil primed surface. The results is muddied flesh tones. I wont use such dark grounds in the future. Plan to bring the ground value back to above a midtone.