Step 1 -- Sketching directly onto a ground or gessoed surface with diluted paint is the most immediate and will give you truer colors. Thin paint with a solvent (turpentine or OMS), or an oil medium that contains mostly solvent and very little oil (or in acrylics, thin with water). The color used for the sketch can vary, but most recommend using neutral browns, such as burnt umber or burnt sienna. Regardless of the sketching colors you choose, it is important to use one that creates a wide range of values. Burnt sienna is a good brown to use, because it will have little affect on colors that are layered on top. Standing away from the painting during the sketching process will allow you to "map" out the composition quickly. Shade in areas and use a paint rag (with a solvent, if necessary) to remove or erase paint. At this point, it isn't necessary to have a meticulous or perfect drawing; in fact, this wastes time, because you will be eventually covering the sketch with additional layers of paint.
Some artists prefer using pencil to sketch out the drawing, although I think pencil can muddy color. In addition, pencils can "tighten" the drawing which slows up the painting process. Tightly rendered drawings will also be lost when you begin to layer your paints, so try to stay loose during this stage.
Shade in areas with thinned paint and don't use charcoal or other dark drawing materials. If the drawing materials used are too dark, they will mix with your paint and cause it to become dark and muddy. If you choose to draw in pencil or another "dry" medium, try fixing the material to the surface with a fixative or liquefy the material by adding a thin oil or thin acrylic painting medium (depending on the type of paint that you use). Binders in the medium help the pigment stick to the surface.
Step 2 (Blocking-in color) -- This may be the most important step in painting. The most efficient way to achieve a good composition and unity in a painting is to initially develop large color areas. For oils, block in areas with paint that has been thinned with an oil medium that contains mostly solvent. In acrylics, use paint that has been thinned with water or a polymer medium. Use only necessary colors. Block in the entire surface, including the "background" and negative areas.
This should be done very quickly and with a large brush. You should be standing during this process so you can get back and see the entire surface area, rather than getting caught up with details. Consider using cool and warm color as well as light and dark to develop space.
Step 3 -- After the overall color plan is completed, the next layers of paint, which might include blending or dabbing can be started. Working from thin to thick, generally, is the best approach. Some painters do a lot of blending. This is achieved by applying patches or dabs of paint over an area before they begin blending in the paint.
Other painters try to build up fresh, unblended dabs or strokes on the canvas. Impasto paint applications may also be used at this stage of the painting, including knife painting.
In oil paint, apply more oily layers of paint over less oily areas (fat over lean). Otherwise, your painting may crack. This problem does not occur with acrylics, because they dry more uniformly and are more flexible. After the initial sketching and blocking in layers, use a medium that has been mixed in advance for the remaining oil painting. This can alleviated many problems with consistency. Avoid mixing pure oil or pure solvent with your paint in these subsequent layers.
Step 4 -- Final stages
Because oily layers should be applied over leaner layers of oil paint, transparent glazes made with an oil medium generally should be applied during the final stages of an oil painting. Glazes can create transparencies in specific color areas to unify the painting. They may be applied in one or more layers over transparent or opaque paint. Using glazes in this way differs from traditional glaze painting discussed below. Because fat over lean isn't an issue with acrylics, acrylic glazes can be applied on acrylic paintings at any stage.
Things to consider
Because oil paint lends itself to blending, it an excellent paint to use if you are working in a realistic style. Oil will often dry in one or two days, however, so it makes sense to finish particular areas in one sitting. If the paint is allowed to dry on an unfinished area, wet paint will have to be reapplied in order to complete the work. An oil painting medium will increase the creaminess and fluidity of the paint and can slow the drying time, if desired.
Using reflective colors at this stage will give a unified quality to the work. This is achieved by "unloading" varying amounts of one color to another color area.
Reflective color use is only one option, however. Some artists prefer working in pure areas of color. This often lends vibrancy to the painting because of contrasts among the larger color areas. Limiting the palette, repeating the colors in the work, or using limited amounts of reflective color will prevent a fragmented, "coloring book" appearance (when areas seem unrelated or simply filled in). Creating powerful value contrasts by darkening and lightening colors will also give the painting "structure" and therefore, create a more unified work.
The danger of adding black or white to color is that the relative purity of the color diminishes. One should also think in terms of contrasts in color, such as complementary colors and other color schemes, to increase the vibrancy of a work.
Perhaps one of the most effective ways of developing space in painting is through the use of cool and warm colors. Cool colors recede in space, i.e., blues, violets, blue green and blue-violet, while warm colors appear to push forward in space, i.e., yellows, oranges, reds, red-oranges and yellow-oranges. The impressionist painters used cool and warm colors to create ranges of depth rather than relying on dramatic darks and lights to create spatial contrasts, as was often done by the traditional academic painters.
Drying time is effected by the amount of oil present in the paint or painting medium. Painting mediums made with a higher proportion of oil will cause the paint to dry more slowly (an often desirable effect). Some colors, such as titanium white or ivory black, contain more oil and therefore dry more slowly. Some manufacturers mix more oil into their paints than others. The thickness of the paint also effects drying time. Oil paints dry more quickly in warm conditions, as well. In the summer, oil painters will sometimes warm their paintings in direct sunlight to accelerate the drying time. Because of all these factors, it is often very difficult to predict the speed at which an oil painting will dry.
Drying can be somewhat controlled by the type of oil used in the medium -- for example, sun-thickened oil dries rapidly and is useful in glazing or transparencies, because layers can be covered very quickly (sometimes in 24 hours). Stand oil doesn't dry quite as fast, so areas stay "workable" longer. Stand and sun-thickened oil are ideal in painting mediums and can be mixed together for even greater control of drying. Mediums made with refined linseed or safflower oil are thinner than those made with stand or sun-thickened oil. Refined linseed oil can be mixed with either of the thicker oils if necessary.
Commercially produced mediums made with alkyd resins make oil paints dry more quickly than mediums made with traditional drying oils. And although alkyd mediums are far less toxic than traditional mediums made with damar varnish and turpentine, quick drying can be a negative when painting in oil. Oil painters like to work wet-in-wet, sometimes for long periods of time. Gamblin Paint, in particular, produces a variety of Galkyd mediums, including Galkyd Slow Dry which provides more "open" time for painting in oil.
Acrylics dry more quickly than oil, so learning how to layer wet paint over dried areas is key to using this medium. To increase the fluidity of the paint, and add to the transparency, add water (no more than 25%) or mix in any proportion of matte or gloss medium. Matte medium has a duller finish than gloss. The type and style of painting will ultimately determine which medium to choose. On very dark paintings, gloss medium will cause the surface to be highly reflective. This can sometimes make the painting difficult to view in some lighting situations.
It is also important to work from thin to thick in acrylics, especially if you want to use impasto techniques. Thick acrylic cannot be removed or changed after it has dried.
Oil glaze paintings -- Directly painting wet into wet with opaque oil became popular beginning with Impressionism and continues into current contemporary painting. Traditional oil glaze painting is an indirect method that was generally more common in academic painting prior to the mid 19th century. It is a technique that involves the layering of transparent color glazes onto a monochromatic underpainting. Artists first complete a thin tonal underpainting usually done in black or brown oil paint. This tonal painting is traditionally called a grisaille from the French "gris" (gray in English). Layers of transparent color (see Pigments for a list of transparent oil colors) are mixed with an oil glaze medium and applied over this fairly accurate and exact underpainting -- allowing the "drawing" to show through the glazes. Many glazes can be layered on top of each other. The intense luminosity of an oil glaze painting is created by light reflecting through the transparent layers of thin glazes.
Acrylic glaze paintings -- An acrylic glaze painting can be created in a similar step-by-step process used in an oil glaze painting. Acrylic glazes can be made with either gloss or matte medium. Small amounts of transparent acrylic color can be added to the acrylic medium. Use limited amounts of water, as it will reduce the transparency and perhaps even make the color cloudy. One disadvantage to using acrylics is that wet polymer mediums lighten the appearance of colors, and the resulting dry glaze can differ dramatically from the wet mixture. More precise glaze colors can be tested, then measured and premixed and stored in airtight containers. An advantage is the quick drying time, so subsequent layers can be painted in a manner of minutes rather than overnight.
Combining oil and acrylic -- Painters can begin a painting in acrylic and complete it in oil glazes. The oil glazes bring a rich, luminous quality to the acrylic underpainting.
Use odorless thinners and soap and cool water to clean oil paint from brushes, etc. Brush cleaners such as Master's Brush Cleaner and Preserver or Pink Soap work well for cleaning oil paint brushes. In addition to cleaning oil brushes, Murphy's Oil Soap can remove wet oil paint from clothing. DO NOT DISCARD OIL OR OIL SOLVENTS IN THE SINK. DOING SO WILL CLOG THE SINK.
Use plain soap and cool water to clean acrylics. Dishwashing detergent or liquid hand soap work fine. Water can be poured down the sink, but thick chunks of acrylic paint should be thrown in the garbage.
Place the brush in the palm of your hand and swish the brush to remove the paint. Run water through the brush and repeat the process until the water runs clean. Reshape the brush with your fingers and store in a jar or can handle-side down.