Under-painting is a transparent “oil sketch” which is made mostly with oil paint and Odorless Mineral Spirits, and which remains underneath the finished painting. It can serve several uses ranging from being a simple composition guide; to being an integral background element visible in the finished painting. It may be a very rough sketch, or a finely rendered image. It’s chief value is in creating luminescence in the painting, since light reflects through it from the back of the canvas; and also for uniting color values in the overall painting by adding a subjective dominant color key.
Under-painting can be tonal, or color blocked. Under-painting has three functions. First, creating some texture, or build on the canvas. Second, putting a color underneath in your painting. Finally, it is a way of seeing whether all the elements balance up and fit your painting. There are three types of under-painting.
This is when the entire canvas has a single transparent color evenly applied, showing through as the shadow color to the painting. This creates luminescent shadows, and tones the entire composition. The greens and whites were painted on top after the brown layer was dry. Under-Painted layer next layer painted on top.
This is when the contrast has been painted with a single transparent color. The white parts are not
white paint: they are unpainted canvas. The lightest areas are lightly stained so not to expose the canvas underneath. Notice how much brighter these highlights appear as more colors are applied. This method can give you very bright and pure top colors and gives you a strong start on developing the form of objects, plus uniting the painting with a dominant color tone.
This is when the contrast is painted, but with the local color of each object or area. This also gives you the bright highlights and jump on developing form, but you can use color composition instead of tonal unity.
Under-painting is the basic sketch for the canvas. When you apply paint as an under-painting, don’t use medium. Thin the pigment with pure Odorless Mineral Spirits. As you paint over in subsequent layers, this will lift off a bit and blend seamlessly with the other layers. If you have an area that you know will remain exposed, it’s best to first do a very light tonal ground to get rid of the stark white canvas. Then apply the under-paint with fresh medium and a brush dampened with Odorless Mineral Spirits. This will bind the pigment adequately. If you use a lot of medium in the first layers of a painting; you risk the outer layers cracking and peeling off as it ages.