The next acrylic paint layer can smear a pencil or charcoal sketch, converting the paint into a muddy or grey tint as it combines with the pencil or charcoal. This is called "acrylic smearing" and it's very common when working on canvas boards. The paint will absorb into the wood of the board, changing its color, and then release itself back out again in a new form. This goes on forever - releasing and re-applying paint - until no more will come out of the tube. Acrylic paints are easy to smear because they're thin layers of paint suspended in liquid acrylic medium. As you apply pressure to the painting with your hand or a brush, the medium under the paint film absorbs that pressure, spreading the thin layer of paint over a wider area. There are two ways to prevent acrylic paints from smearing: use thick layers (about 1/4 inch or greater) or work quickly after applying each color. Thick layers of acrylic paint force any unsquished areas of color beneath them to combine together. So if there's a dark spot where you don't want it to blend, such as a tree against a sky background, you'll need to let some time pass before adding another color layer. This gives the first color enough time to dry before adding more.