Varnish is more than simply a layer to protect your oil painting from ultraviolet light, pollution in the atmosphere and abrasion. It will also bring out the brilliance in colors, making them sparkle. Varnish is made from Damar resin dissolved in a mixture of turpentine and other organic solvents. Keep in mind that oil paint dries from the outside in; when it dries, it forms a surface skin first. During the drying process small pores form in this surface skin. The surface of the paint may feel dry, but under the skin, the paint is still wet, no matter how thin the coating is. It takes at least 6 months drying time for the dried oil paint film to become dry enough that it won't be re-dissolved by mild solvents such as turpentine. When a varnish is applied to a reproduction art oil-on-canvas painting before the 6 month drying time, the varnish fills the pores of the oil paint film, The solvent content of the varnish can easily re-dissolve the incompletely dried oil paint at the bottom of the canvas and cause all kinds of problems - such as wrinkling of the oil paint film, cracking, and yellowing of the color.
• Clean the painting so it's free from dust and dirt. Lay the oil painting flat, then dampen the surface a bit of cotton wool with clean water.
• Dry the oil painting with another bit of cotton wool. With your fingers, gently remove any cotton fibers that have been caught in the oil paint.
• Leave your painting to dry for several hours.
• Choose from high gloss, satin or matte finishes. Varnishes may be brushed or sprayed on. Use a flat bristle brush to apply the varnish. If you don't want your oil-on-canvas painting to be too shiny, use a matt varnish rather than a gloss one.
• With the painting flat, work from the top to the bottom, applying the varnish in parallel strokes from one edge of the oil painting to the other. Always work in the same direction. Try to have the same amount of varnish on the brush for each stroke so you put equal amounts of varnish on all parts of the oil painting. Always varnish the whole of the oil-on-canvas painting in one go.
• When the first coat of varnish is dry, apply a second coat at right angles to the first. This will give you a good, even finish.
• Leave the painting flat for at least 10 minutes after you've finished varnishing to stop the varnish running down the oil painting. Then prop it up against a wall to dry, face inwards.
• To test whether the varnish is dry or not, touch the edge of the oil painting to see if it's still tacky. It should dry within a day or two, depending on the weather.