Students new to painting are usually hesitant and indecisive in their work, which usually means keeping your paint thin because you’re not sure how to mix colors. This is a problem because paint thinned with a lot of thinner or medium is weaker color. Oil paint is opaque and works best when mixed and applied in thick strokes, not thin washes. You learn more about color and painting techniques with thicker paint. It also helps you be more decisive with your decisions. It’s harder to be tentative with thick paint. Here is an example of thick paint and how it influences color by Emile Gruppe.

x