Oil paints can at first seem a little bit mysterious. They don’t behave like other types of paint in terms of handling or drying time, and a huge range of materials and techniques have been developed throughout the history of the medium. As complex as oil painting may seem, though, at the core of the craft are a few fundamental principles. Learn them, and you can begin to unlock the descriptive power and expressiveness that makes oil paint the favorite medium of so many artists.
Where to Begin
Some artists begin by covering the entire canvas with a thin veil of a naturally fast-drying earth color like raw or burnt umber, but others jump right in full-palette. The main advantage of starting with a monochrome (single earth color) is being able to establish lights and darks before handling the overall color scheme. Starting full palette on a bare primed canvas keeps colors bright and vibrant.
Fat over Lean
Success in oil painting depends to some extent on sensible layering. "Fat over lean" is the traditional principle that recommends the first layers have less oil than each subsequent layer. Also, faster-drying layers should be applied first, followed by slower drying ones. This ensures good adhesion between layers and prevents defects like peeling, crazing or wrinkling.
A Painting Medium is a fluid or gel that is mixed with the paint to improve brush movement, make paints more workable and translucent/transparent, and impart uniform gloss. If you’ll be using a traditional oil based medium (usually a mixture of oil, thinner and sometimes varnish), in the very first stages, colors should be thinned only with Odorless Mineral Spirits (OMS) so paint doesn’t become too “fat” or oily.
Introduce oil medium after the initial layer has been applied. Alternately, if you choose a fast-drying, ready-to-use alkyd medium instead of traditional oil mixtures, it's best to use your medium from start to finish or just in the first stages. Whatever medium you’re using, try to use the minimum effective amount and avoid overthinning colors.
Measuring by Thumb
Artists are often observed squinting at an outstretched thumb. This technique is used for measuring and comparing the apparent heights and widths of elements within the picture. Try for yourself: Hold a paintbrush upside down so your thumb rests near the brush handle sticking out of your fist, and extend your arm fully like you're giving a "thumbs up" with the brush handle. Close one eye and sight against the brush handle, visually measuring between the tip of the brush handle and the top of the thumb. Extend your arm fully without bending the elbow for accurate comparisons each time. If your elbow is bent, the length of your arm will not be consistent between measurements. Compare elements in the subject to achieve accurate proportions; for instance, in a human figure, count how many heads tall the torso is. This rough comparison will help keep proportions in a plausible range.
Working with Color
Work with the brightest hues first, then mix toward neutrals (browns, grays). Blending two or more bright colors can make a more dull mixture, not a brighter one, so be conservative at first when mixing, especially with complementary colors. Adding white to mixtures can increase apparent brightness, however, especially with dark, transparent colors like phthalo blue that can only achieve full brightness by adding white to "open up" the pigment.
A good way to understand "neutral" or "earth" colors (also called browns and grays) is that each is a more dull version of a brighter color that can be named as a primary or secondary color of the spectrum, so challenge yourself to “name” neutrals as you mix them, according to the color wheel. Colors should be mixed on the palette as much as possible-not on the canvas to keep clean, bright hues. Mixing with the palette knife rather than the brush makes it easy to produce enough of a single color all at once when a broad area needs covering. Mixing with the knife also saves wear and tear on brushes.
In between sessions, sometimes colors can “sink in” and appear dull as paint begins to dry. One way to remedy this is by “oiling out” before the next application of fresh paint. This involves applying an extremely thin application of medium, just enough to restore the dry paint to its wet appearance and promote fluid brush application. Take care to rub in just enough medium to achieve desired effect, not so much that the canvas feels greasy. If you’d like to revive colors without wetting the surface, apply a coat of retouch varnish instead.
Step Away from the Canvas
When you stand at a fixed distance from the easel for a long time, it’s easy to lose your grasp of proportion. Periodically step back from the canvas and take breaks at intervals to stay fresh. If the canvas becomes completely saturated with wet paint to the point where it's hard to add more without disturbing the wet layer, it's time to wrap up for the day or move to a different canvas.
An important part of painting is learning what makes a picture “finished”. While there are always exceptions, in general, unfinished is better than overworked. The last passages on a picture should be carefully considered and deliberately executed. After the major statements in color and value have been made, the final stage of painting should unify everything, fine-tune edges and make the highlights "pop". The consistency of the paint in the final stages is very important, because the brushstrokes will communicate themselves more in this layer than in any previous one.
Drying Time and Varnishing
Unlike acrylic which dries relatively quickly by evaporation, oil paint cures through oxidation over a period of months, during which time dimensional and chemical changes are taking place. For best results, a final varnish shouldn’t be applied until paint is completely dry, because wet varnish can intermix with still-drying layers, or changes in the paint might spoil the varnish through wrinkling or "orange peeling". Retouch Varnish can be applied to paintings dry to the touch but not yet completely cured; when paint is completely dry, retouch can either be replaced or top coated with a final picture varnish depending on the type used.
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