Oil paints require a lot of patience from the artist. Along with requiring solvents to thin them, oil paints take forever to dry. When selecting the solvents to thin oil paints, avoid industrial ones, because they are more noxious than solvents meant for artistic purposes; the right kind are available at arts and crafts stores or online. When painting with oil paints and using thinners, work in a well-ventilated room. Artistic solvents also evaporate completely in ways that prevent negative effects on your paint colors.
Oil Painting Solvents
Artists can choose from turpentine -- made from conifer tree sap -- odorless mineral spirits, linseed, and safflower or essential oils to use as thinning agents for oil paints. Turpentine can cause headaches when used without proper ventilation, because of the chemicals used during the sap extraction process, which is why many artists prefer odorless mineral spirits or oil-based thinners. Mineral spirits are a petroleum-based solvent, also known as white spirits. Essential oils give you a natural, less toxic and pleasant-smelling thinner or solvent for working with oil paints.
Tip: Linseed oil can add yellow spots to lighter colors, so choose a different thinner for pale colors.
Natural oils fall into three categories: non-drying oils, essential oils and drying oils. For oil paints, avoid the non-drying oils -- olive oil or vegetable oils -- because your painting will never dry. Drying oils consist of natural plant-based oils such as linseed oil from flax plants, safflower or walnut oils. Most oil paints are made from a combination of pigments particles and linseed oils, so linseed oil also thins the paint when more is added. Essential oils like rosemary or lavender made from a distillation process evaporate in the same manner as mineral spirits and turpentine do when in contact with oxygen, which helps to dry the oil color on the painting as well.
Thinning Oil Paints
The first rule of thumb in oil painting is fat over lean. The leaner, or thinner, colors go on first so they dry faster; the fatter, less thinned paints are used on the top layers. Take a bit of color from a blob of oil paint on your palette and mix it with your desired thinning agent. For the first layers, use dime-sized amount of color and plenty of solvent. Thoroughly mix the solvent and the bit of paint together by blending them with the palette knife. Once the paint is thoroughly blended, brush it onto the canvas following your lightly pencil-sketched drawing.
When you don’t abide by the fat-over-lean method of oil painting and use thicker paints on bottom layers, the top layer can dry out too fast and result in shrunken and cracked paint. Because oil paints take time to dry, don’t paint the next layer until the first layer has thoroughly dried to the touch. To clean up brushes and tools, use the same solvent you used to thin the paints.
A nice factor of working with oil paints is that you can paint on wood, canvas or basically any surface, as long as you prime it first. Before you can add oil paints to your chosen medium, use an oil painting primer to cover it completely. Primers work especially well on fabric canvases, because being acid-based, oil paints can eat away at them over time. Applying an oil-painting primer -- usually white -- to the canvas and allowing it to dry preserves your painting against deterioration. Primers also prepare wood and other options by smoothing out the imperfections in the surface.