Painting mediums can alter the flow of paint, making it easier to achieve fine details.
Painting mediums are either mixed with oil paint or used on top of a finished oil painting. Mediums mixed with oil paint alter the flow of the paint, making it thinner or altering the drying time. They can also make the paint glossy, matte, opaque or transparent. Varnish Mediums are applied at the completion of the painting to hasten drying time or to create an even sheen over the entire painting.
Thinners
Thinners can be added to oil paint to increase flow.
Use thinners to dilute oil paint and to clean brushes and palettes. Turpentine is the most common thinner; it is mixed with paint to help with paint flow. When paint is watered down, thinners can create a wash, which is a thin mixture applied in layers to the canvas.
Low-odor solvents are a good option, because they are less harmful and have fewer fumes. Low-quality thinners found in hardware stores may be cheaper, but can also alter the final color of the paint as it dries. White spirits and mineral spirits also work as paint thinners. They should be used in a well-ventilated area.
Oils
Oil mediums can be added to oil paint to increase thickness.
There are a variety of oils on the market; each has its own benefit and drying time. Linseed oil, which is made from flaxseeds, is the most common. It comes in a pale to light yellow color and when mixed with paint helps to create thin, transparent layers of color. It usually takes 3 to 5 days to dry once it is applied to the canvas or board. Cold-pressed linseed oil is a higher quality than refined linseed and may dry faster. Linseed oil may yellow pale colors like white and light blues.
Stand oil is a thicker oil, and is ideal for glazing. It may need to be mixed with a thinner such as turpentine to produce smooth finishes without brushstrokes. Results appear similar to enamel paint. Stand oil may remain tacky for some time, as it is slow to dry.
Poppyseed oil is ideal for mixing into pale colors. It is less likely to yellow than linseed oil. When added to oil paint, the consistency it creates is similar to butter. This is a good medium to use to work wet on wet, though it is only recommended for top layers of a painting because it takes so long to dry. The drying time may be up to 7 days.
Walnut oil is an expensive oil that dries with less yellowing than linseed oil. This is the oil that Leonardo da Vinci used on the "Mona Lisa" painting. In raw form, walnut oil is a pale yellow-brown. It is a very thin oil that helps to create fluid paint, which is good for applying thin, transparent layers. Walnut oil must be stored in a cool, dark environment so it will not turn rancid. Drying time for this oil is up to 5 days.
Varnishes
Varnishes can help create an even overall gloss to a finished painting.
Varnishes are used for the final layer of the painting process. They generally give a cohesive feel to a painting that may otherwise appear shiny in some parts and matte in others. Gloss or matte picture varnishes are mineral spirit-based and will not yellow the final painting. Matte and gloss varnishes can be mixed together, resulting in a satin finish. If you need to remove the varnish to go back in and correct a painting, turpentine or white spirits can can be used to remove varnish.
Retouching varnishes can be applied over a painting that is dry to the touch. This is a thin, liquid varnish that is helpful in getting rid of dull spots on a finished painting. This varnish may also be removed with turpentine or white spirits or left on as a final varnish coat.
For a very fast-drying varnish, use Damar's varnish; it dries within a few hours and results in a satin or medium-finish gloss. It too, is removable with turpentine or white spirits.
