I finished a charcoal drawing and want to paint over it with a clear coating (of acrylic gesso). For that I first used a fixative so that the drawing will not smudge when I apply gesso with a Brush. However, it seems that the fixative does not work the way I intended. The charcoal is still (after a couple of coatings with fixative) not fixed, meaning that when i slightly rub on the surface it leaves a trace.
What might be the reason for my drawing not being fixed completely?
Could it be that the fixative is not good? Do I have to apply a couple more coatings of fixative?
Is it even possible to fix a charcoal drawing in such a way that it is unsusceptible to rubbing or brushing? If yes, how can I achieve this?