While there are some that would state that art works
do not need to be varnished, varnishing does contribute a number of beneficial
elements to a painting.
Consistency of gloss level: Most finished artworks have a patchy
inconsistent gloss level that is resultant from the varying number of layers
of paint over the surface of the canvas as well as the gloss levels that
are inherent in each of the pigments. (the shape of the pigment can reflect
light in different ways to other pigments). Varnishing lends itself to give
a more unified finish to the artwork which is less distracting to the eye
than an un-varnished piece.
Consolidation of work: Carefully applied varnish will ensure stability of media such as pastel and charcoal. Media such as watercolour, gouache and tempura can be "locked" into place.
Saturation of colours: Varnishes tend to increase the saturation of colours making them stronger and more vibrant.
Protection: Possibly the most important function
of a varnish is to protect the work. Media such as acrylic are thermoplastic
and tend to become softer and stickier as temperatures increase causing
dirt pick-up from the environment. While most artists varnishes are also
thermoplastic, they are typically made from harder, less sticky polymers.
Some varnishes also incorporate UV blockers and absorbers, which reduce
the amount of colour destroying UV to penetrate through to the media.
Ease of Cleaning: varnish provides protection to the finished artwork so that it may be easily cleaned without risk of damage to the underlying work.
There are a number of different types of varnishes available on the market ranging from traditional materials such as mastic, copal and damar through to more modern materials such as polyurethane, waterborne acrylic emulsions, and solvent based solution acrylics.
Ideally an artist's varnish should be removable in case there are issues with the varnish application and it needs to be stripped off and redone or if in years to come, conservers need to undertake restoration work. It is also very important that the varnish itself does not degrade. Traditional copal and mastic varnish are not easily removed and while damar is removable it trends to yellow very significantly and should therefore be avoided.
Modern aliphatic polyurethane's are claimed not to yellow and are available as waterborne systems or solvent based applications. While there are some significant benefits in utilising polyurethane's such as very high gloss, excellent levelling and low foam formation there are significant negative elements such as narrow re-coat adhesion window (polyurethane's are so good at there job it is difficult to get anything to bond to them after about 7 days curing), and most importantly they are not strippable.
Polymer Varnishes are typically based on acrylic technology and are water based. The dominant advantage of polymer varnishes are they are easier to use, without the odour and hazards that are associated with solvent based varnishes. Polymer varnishes are suited most to acrylic media and should not be used for work intended for exterior display. A well designed artists varnish is one that can readily be stripped from the surface without affecting the underlying artwork and caution needs to be taken to ensure that the product used is removable as there are some products available in New Zealand sold as artist varnish that are not removable.
Solvent-based artists varnishes in the main can loosely be called Solution Acrylics. There is some ongoing discussion as to what is the better system whether a low molecular weight (LMW) such as Gamblin's Gamvar varnish or at the other end of the scale Golden's high molecular weight (HMW) MSA varnish. From a technical perspective there are strong advantages in each of the arguments. LMW tend to be slightly softer than HMW polymers and offer greater resistance to moisture, better resilience to UV, greater colour saturation and have a higher gloss. All in all there is very little in the way of observable difference for the artist between one product type and the other and most solution acrylic artist varnish can be roughly considered the same thing regardless of brand.
Solution acrylics have been around for a long time and versions of them are used successfully in a wide variety of high endurance type coatings from stamped concrete and slate protectors to glazes on concrete roofing tiles. There is a wide variety of solution acrylics available with many of the more industrial products cut in very agressive solvents such as xylene. These products are unsuitable for art works as the solvent may attack the paint film.Artists varnishes are usually made from less aggresive low aramatic containing solvents which while having pitfalls is less toxic and more suited to artists applications.
Advantages of solution acrylic based varnishes are they have proven the test of time in some very harsh conditions, easy to apply, suitable for protecting external artwork. are suitable for both acrylic and oils and solution acrylic varnishes are easily re-dissolvable in selected solvents, which makes them the ideal choice for artwork.
Almost all artists spray varnishes are a diluted version of Solution Acrylics and are within reason the same thing regardless of brand. Some versions incorporate UV blockers and absorbers but aside from that they are either LMW or HMW acrylic based polymers that have been heavily cut with solvents such as acetone and propellants.
Dollar for dollar, spray varnishes are the least economical way to varnish a painting. However they do offer some distinct advantages and application properties. For example they are ideal for isolation coats to protect materials such as imitation gold leaf prior to applying water-based polymer varnishes as polymer varnish tends to oxidise the copper in the leaf. Spray varnishes are also excellent for building up light coats of varnish to "lock in" watercolour, gouache, pastel and charcoal.
Typically the base polymer or solution acrylic comes
as a gloss product that is then adjusted by the producer to make either
satin or matte finishes. Matting agents are used for this purpose and are
either silica or waxes. In both cases, the matting agents reflect white
light which can be a cause of decreased colour saturation and in worst cases
a milky bloom on the surface of the painting if basic application principles
are not adhere to.
It is relatively easy for an artist to adjust gloss levels in solution acrylic
paint on varnishes by simply stirring in a wax medium to the gloss varnish.
However, water based varnishes require silica based matting agents which
are not readily available over the counter.
Isolation coats can provide specific advantages in the methodology of varnishing. As mentioned above they can serve to lock in otherwise soluble or easily moved media much like a fixative is utilised. However of recent times, one of the main areas isolation coats have been promoted is to create an immovable barrier between acrylic media and the varnish to protect the acrylic paint if the varnish ever needs to be stripped off. While there are some considerable advantages in utilising the isolation coat technique such as to provide uniformity of gloss and reduce the porosity of the surface prior to varnishing, there are negative elements and some debate as to its necessity in most acrylic artworks.
Oils:
The generally recognised rule of thumb is to allow 6 months of curing prior
to varnishing. However, as this is only a general rule considerations such
as medium types utilised, paint film thickness etc. can shorten or dramatically
lengthen the time of cure. For example a strongly impasto effect may require
12 months or more before there is sufficient oxidisation of the paint film.
Acrylics:
There was a time not that long ago where it could be said that most acrylics
pieces can be varnished within about 7 - 14 days of completion of the work
depending on the thickness of the paint, the ambient humidity and temperature
conditions. A traditional acrylic paint cures by loss of solvent (water
and coalescing solvents) with the sticky acrylic particles forming long
polymer chains as the solvents dissipated.
However, several more recent "innovations" in acrylic paint technology
designed to keep acrylics open and workable longer have changed the standard
rules somewhat in as much as these paints can retain elements of solubility
well after 30 days of curing. Caution needs to be exercised until such time
as the paints manufacturers can state definitive curing times.
Surface preparation:
In all cases the finished piece must be dry, sufficiently
cured and free from contaminants. Oils that has been stored waiting on sufficient
curing should be lightly washed to remove dust and other contaminants and
allowed to dry completely.
Note that acrylic paints contain surfactants of various kinds that can migrate
to the surface of the work and create an adhesion barrier. Work that has
been set aside for 2 - 3 weeks may have surfactant based contamination and
should be washed with warm water and allowed to dry completely.
Isolation coats should not be applied to acrylics
until you are sure the paint is dry. Allow 2 - 7 days of curing prior to
varnishing.