Gosh, we have had a lot of questions about varnishing lately! Let's briefly explore the intricacies of that final protective
coat for your painting.
Varnishes are solutions of natural or synthetic resins
that dry with relative transparency when spread thinly on a surface. Solvent-based varnish is the most popular type of artists'
varnish and includes synthetic resin in water (Golden and Liquitex acrylic varnishes), synthetic resin in
spirits (Soluvar and Gamvar) and natural
resins in turpentine (damar and mastic varnishes). The latter have a
tendency to yellow over time. Solvent-based varnishes generally dry
by evaporation and are, for the most part, reversible. Some traditional
oil painting varnishes, such as copal varnish, are
natural resins melted in heated oil and combined with driers and turpentine.
These can be much more difficult to remove due to different drying processes
and are also prone to darkening.
The primary functions of a picture varnish are to protect the painting
from dirt and other soilents and to give the painting an overall consistent
finish. The varnish should be flexible, clear and removable. Oil painting
mediums such as Liquin or Galkyd are
inappropriate for use as a varnish as they tend to yellow, are inflexible
and are not removable. Also, contrary to what some believe, applying
a drying medium over a wet oil painting will not cause the paint film
to dry faster; rather it seals in the paint, preventing proper oxidation,
which can lead to cracking.
Watercolour and pastel paintings
are traditionally not varnished; framing under glass is recommended.
However, certain spray varnishes could be appropriate. Acrylic paintings may be varnished with a water
or spirit borne varnish soon after the paint film has dried. Oil paintings should not be given a final varnish until
the film has properly oxidized. In the case of a thinly applied paint
this may be as little as a few weeks time.
Heavily impastoed oil paintings made without the benefit
of a drying medium added to the paint may take a year or more before
they are satisfactorily dry enough to varnish. Retouch varnish should
be used as a temporary finish.
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