To varnish or not to varnish...that is the question.
I say varnish.
As with solvent not all varnishes are created equal therefore there are differences between them.
First of all an oil painting must be dry before applying a varnish. How long? Depends on how thick the paint has been applied, what mediums have been used, atmospheric conditions...you get the picture.
As a general rule an artist should wait at least a year before applying a final varnish. We should wait at least 3 months to put a protective coat of retouch varnish on a painting, and once again this depends on many factors.
As a general rule a varnish should do the following:
-A varnish should be transparent and without colour or tint.
-A varnish should be reversible for later cleaning and restoration.
-A varnish should have the appropriate "gloss" for the work in question.
-A varnish shouldn't yellow with age.
-A varnish should be applied in a thin layer so as not to disturb the textural effects of the painted surface. A thin coating of varnish is less likely to crack. Too thin can actually be removed too easily and potentially permit damage to the surface of the painting.
-A varnish should be able to expand and contract in different environments especially when the painting is on a flexible surface like canvas. The varnish must remain flexible for it's lifetime and not become brittle.
-A varnish must protect the painting from atmospheric contaminants and impurities like dust and other substances found in the air.
Acrylic paintings should be varnished too with the same considerations. An artist doesn't have to wait a year to varnish an acrylic painting however if one considers that it takes about one month for acrylic gesso to dry completely, the same consideration should be taken for a finished work. How thick the paint has been applied should also be considered. Trapping humidity under the surface of any varnish whether it be oils or acrylics may cause a condition called "bloom" where the varnish actually becomes cloudy and looks like the painting has a cataract...yuck!
The secret to this stage of your work is patience. I was told as a young student that an artist should live with his or her work for a while before letting it go out into the world. I see far too many painters letting their paintings go without letting them dry properly or getting a basic protective coat of varnish. For example I saw some very impasto oil paintings for sale in early January dated 2008...the paint couldn't be dry and there wasn't any varnish...and yes they were oil paintings!
Very poor craftsmanship indeed!