Acrylic is a fast-drying medium that resembles oil paint in many ways. When dry, the two can be difficult to tell apart. However, acrylic is differently rendered than oil paint because it is water-based rather than oil-based. Painting a portrait with acrylics can be challenging because of its quick drying time. Color blending, an important part of creating portraits, is difficult with acrylic.
Preparation
Prepare your canvas for painting. Because you are working with acrylic, it is not necessary to prime your canvas (paint it with gesso), but most painters prefer it because an un-primed canvas tends to absorb more paint.
Sketch the subject multiple times before beginning the actual painting. Prepare for the painting by sketching the outline of the subject onto the canvas. Work from a subject that is real, rather than imagined. When choosing a subject, keep in mind that if you choose to work from a live subject, you will face certain challenges. These challenges will include the changing light conditions of ordinary daylight, a subject that is incapable of remaining utterly still and the subtle alteration of poses between one sitting and the next. Photographs make good subjects, but be sure to choose a photograph that is high resolution.
Blending Color and Form
To blend your colors---and when painting shadows---you will want to paint quickly, add acrylic retarder, water down the paint, or some combination of all these. Most portraits will require quite a bit of blending. Begin by painting a layer of flat colors that cover the entire canvas. Build from these colors with highlights, low lights and details.
Once the paint is dry, if you need to add shadows or highlights to something already on the canvas, you will need to thin the paint used for the addition in order to allow the paint underneath to show through. This will make for less jarring transitions between the shadow or highlight and the original color. Alternatively, you may mix the base color again and apply it again to the canvas, then add highlights and shadows to the base color while it is wet.
Time Frame
Do not try to complete a portrait in one sitting, even if you are painting from a photograph. Returning to the painting multiple times will give you a fresh perspective and allow you to catch errors.
During the time in between sittings, the acrylic paint will completely dry. In order to make changes, you may find yourself struggling to blend the new additions with the old image. Again, to blend the new paint with the old, you may be required to thin the new paint so that the old paint will show through. You may also find yourself needing to repaint entire sections of the the portrait in order to match the old colors with the new. Acrylic can be painted in layers again and again, nearly endlessly, without concern that the layers underneath will crack the thinner layers on top (unlike with oil paint).
If you find yourself unable to resolve your errors, try flicking your eyes very quickly back and forth between the subject and the painting to compare the difference in shapes between the two.
Paint Application
Acrylic paint can be watered down and applied to the canvas in transparent layers, or it can be applied to the canvas in textured blobs and thick strokes. These techniques can be mixed on one canvas. Mixing these different applications of paint on the same canvas can add interest to your portrait.
