Folks are often a bit confused about varnishing when it comes to acrylic paintings: Should I varnish? Use a solvent-based or polymer medium? Gloss or matte? And by the way, what's the point of varnishing in the first place? Let's address those questions in reverse order.
What’s the point of varnishing in the first place?
A varnish is the top-coat of material that is the very last thing to be applied to a painting. Traditionally, the varnish layer is composed of a transparent medium that is painted on with a soft, flat brush or sprayed from an aerosol can after the painting proper has been completed and is thoroughly dry.
A varnish serves as a protective layer between the surface of the painting and the outside world, so that dirt and scratches tend to collect on the varnish rather than the painting itself. That way, when the painting gets dirty you can just remove the layer of varnish—leaving the painting intact—and apply a new clean one in its place. Likewise, surface scratches will be removed with the varnish layer.
A varnish also creates a uniform layer on the painting. The different pigments used in acrylic paints may have very different levels of gloss inherent in their composition, resulting in a patchwork appearance on the painting. For example, an earth color like burnt umber may have a very matte appearance, while a synthetic pigment like quinacridone may be very glossy.
Should I use gloss or matte medium?
Gloss is the traditional and most often used type of varnish, and it the colors underneath to create greater richness and depth. However in contemporary art some paintings call for a more matte appearance or, something in between gloss and matte, a satin varnish. All the different levels of gloss are available in varnishes and they can even be mixed together to create different effects.
Should I use a solvent-based or polymer medium?
If you’re interested in varnishing your paintings to protect them, then you need to use a solvent-based varnish. Some great examples are Soluvar by Liquitex and Golden's Spray Varnish, all of which come in gloss, satin or matte finishes. These varnishes use petroleum distillates (mineral spirits) and resins as a base. These elements adhere to the surface of the acrylic painting like glue, but without creating a permanent bond. Solvent-based mediums can be removed using a solvent like Turpenoid, Gamsol or Grumtine with a cotton ball or other very soft applicator.
Since the acrylic painting itself does not react to these solvents, the painting will not be harmed when the varnish layer is removed. Of course the down side to using a solvent-based medium is introducing toxic fumes into your studio that must be ventilated at all times. For those of you who are allergic to mineral spirits, these varnishes will not be an option.
Because of the different chemical composition and the generally brittle nature of the solvent-based varnish, it will need to be replaced eventually, whether or not it gets dirty or scratched. If it is not replaced, the varnish layer may eventually begin to crack or flake off. It may take 50 years to happen, but it will happen. The acrylic painting is, by its own nature, tough and flexible, and able to withstand some pretty harsh treatment. Therefore you may decide that extra protection from a varnish is not necessary and not to deal with the hassle of replacing that solvent-based varnish every couple of decades!
In that case, a polymer medium—usually just called a gloss medium or a matte medium—will serve well to create a uniform surface over the whole painting. A polymer based varnish—such as Liquitex' Gloss Medium & Varnish—will bond with the acrylic painting underneath on a molecular level, effectively becoming part of the painting, and it can never be removed.
However, polymer mediums are not varnishes in the true sense of the word. They are actually just the same medium that the acrylic paints are made out of, made to be transparent without any pigment in them at all.
One very important technical detail must be noted here that may save you from crying later on: DO NOT over-brush a polymer medium when varnishing a painting! When you brush vigorously back and forth with a polymer medium it traps tiny air bubbles inside the plastic (this is usually called "foaming"). Because of the fast drying time of the polymer can become a permanent part of your painting! These bubbles will create a cloudy film, obscuring the colors underneath, and will look generally unpleasant. You won't notice this effect with acrylic paint because the color of the pigment obscures the cloudiness created by the bubbles, but since the varnish layer is transparent, they become all too apparent. So, when applying polymer medium as a varnish, brush it on in smooth, single strokes and don't scrub it!
The best way around this problem is to use polymer medium with a retarder solution, which will slow down the drying time sufficiently to allow the bubbles to escape if you do get a bit crazy with the brush. Both Liquitex and Golden make retarders and additionally, both companies make a product called Glazing Medium. Glazing Mediums are polymer mediums specifically with an appropriate amount of retarder already added. This is, in our humble opinion, the safest way to varnish with polymer medium.
To Varnish or Not to Varnish?
In conclusion, should you varnish? In most cases the answer would be yes. Varnishing has been a professional practice in painting for centuries and most clients will expect their paintings to be appropriately protected. Plus, the traditional glossy varnish is one of the things that potential buyers will expect to see on a painting, so if it's not there they may feel that the work is somewhat amateurish. That said, you may have a unique vision for your work that would be ruined by applying a varnish and you should never compromise on the things that make your paintings unique! Let your intuition be your guide . . . as it always should be!
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