Remember to varnish! Never place any object against a curing oil painting.
This is an example of an unprotected and nonvarnished oil painting. Notice the “fog” and surface damage on the woman’s face in the portrait shown here.
Paintings and drawings need protection from dust, dirt, pet hair, and especially humidity. Some artworks take well to framing under glass, but acrylic paintings and oil paintings need to be varnished for protection from these hostile conditions.
When acrylic paints were first invented, people believed that they would not require any efforts for additional protection. However, art conservators discovered, with the aid of microscopes, that the acrylic painting surface had a top porous layer allowing dirt, dust and oils to penetrate and embed within the painting surface. They saw that the porosity of acrylic paints made it difficult or impossible to easily clean portraits without some forms of damage.
Oil paintings are not only vulnerable to dirt, they are also vulnerable to humidity. I have observed the fate of an unvarnished oil painting, wherein its extended exposure to aggravants and humidity an opportunity to penetrate the surface of the oxidized oil paint, and an ensuing lipid migration ruined and compromised the visual aspects of the painting (see above photo as reference).
Lipid migration occurs in situations when humidity rehydrates dried layers of oil paint underneath the surface layer. The resulting damages can appear as blistering, weeping, or sagging on the surface of the picture. The oil painting can become shiny and sticky or smear.
Varnishing the front of any oil or acrylic painting, when it is fully cured, and sealing the reverse side of any hardboard panel with another coat of acrylic or gesso, are excellent ways we can reduce risk of damage to fine paintings from potentially harmful environments.
Acceptable varnishes can be glossy, satin, or matte finish and they can be brushed or sprayed onto paintings. Varnish can significantly extend the lifespan of many paint varieties. Along with varnishing, a dehumidifier used in humid climates can also make a more favorable environment for storing artwork.
Charity Goodwin, Artist