What are the advantages and the disadvantages to an artist varnishing his/her acrylic painting?
Advantages:
True picture varnish is a removable coating and offers UV protection. The protective coat of a varnish takes the abuse from grime, handling and shipping, evens out sheen, and seals more porous areas such as gesso.
Disadvantages:
A varnish may change appearance for the worse. Single coats of varnish can even out the entire sheen, which may not be the intention of the artist and the artwork loses something aesthetic. Sometimes varnishes can be difficult to apply: running/sagging, brush strokes, foam bubbles. Some may not be so removable later because of a self-crosslinking action.

Why should I varnish my artwork?
The age-old question (at least here at GOLDEN) "To varnish or not to varnish". Let me give you our take on this debate. To keep it simple, we'll just discuss acrylics.

The acrylic resins, polymers, etc. used to make GOLDEN Acrylics are meant to be durable and last hundreds of years. The pigments we use are the most permanent and lightfast in each chemical classification. The resulting paint, mediums, gessos, etc. are therefore archival. So why varnish?

While the acrylics are durable films, they are also porous. When the water evaporates from the paint, it leaves microscopic "worm holes". The tunnels stay in the film after it dries, which is great in terms of moisture passing through the films without loss of durability and adhesion, but bad in respect to dirt and dust accumulation over the years. Even if you are good about cleaning your paintings with dusting and wiping down, some amount of grime gets trapped in these pores, and over time this accumulation can discolor the artwork. Subsequently it will take many hours of conservation to remove the dirt.

Many artists do not varnish because:

• It's difficult.
• It's another step they have to do.
• It may not turn out the way they wanted it to
• It can change the sheen to a more uniform finish, which detracts from the original artistic intentions.
• It can alter the artwork negatively.
• It's not important enough to the artist "It is the gallery's/customer's responsibility, not mine"

So why varnish?

• GOLDEN varnishes are removable, so the dirt and dust retention removes quickly and easily and isn't trapped in the acrylic layer.
• Varnishes have UV inhibitors, which help slow the fading of the pigments.
• Varnishes are the best way to control sheen. If you use matte products (paints, gels, etc.) you begin to develop a foggy or hazy effect. Glazes are especially prone to have this problem. However, you can apply one thin coat of matte varnish and have a very flat sheen, but still retain depth of color.
• If something goes bad during shipping or on display, say a kid with a sharpie permanent marker, you can strip the varnish off along with the graffiti and apply a new coat. If it's in the acrylic paint, then it's very difficult to remove without damage.

In conclusion, We can see validity to both sides. We personally like to varnish because it almost always makes the colors more intense and consolidates the painting. However, applying a varnish would ruin some pieces, as they would lose some character.