One of the most perilous realms of uncertainty in regards to painting is a timeframe for varnishing a finished work. Different sources communicate very different timetables. For example, Winsor&Newton states, "Try to avoid the temptation to varnish the painting immediately after completion. The colour needs to be completely dry and we recommend waiting at least three months before varnishing."
Natural Pigments on the other hand, shares a more commonly touted timetable, “The best practice is to allow the bulk of the oil paint to cure (polymerize) before applying the varnish. The typical advice to wait 6 to 12 months for most paintings before varnishing makes perfect sense. This is especially true for polymeric varnishes, such as those made of acrylic resins.” Some sources, like the aforementioned Natrual Pigments, may inadvertently add to the confusion by also stating on their site regarding varnushes that “Timetables are potentially hazardous.”
So let’s see if we can clear up some of the confusion by looking to two individuals that understand the materials quite well and hold a decent grasp of some of the pragmatic concerns of today’s painters.
First let’s look at George O’Hanlon’s (Natural Pigments) full response regarding timetables to better understand his position and dispel potential confusion:
"Timetables are potentially hazardous, so we recommend testing the paint film for “hard-dry” before applying varnish. Retouch varnish can typically be applied immediately after the paint is hard-dry.
Final picture varnishes, on the other hand, should be applied at least several months after the paint is hard-dry, but this can be shortened depending upon the type of varnish applied, such as a low molecular weight (LMW) resin varnish. Polymeric varnishes, such as Golden MSA Varnish and Liquitex Soluvar, should be applied after this waiting period to ensure that the paint has sufficiently cross-linked to the point that it is not “contracting” aggresively.
Here is a modified test for “hard-dry”: Exert downward pressure (without twisting) with the end of the forefinger on the paint film. Lightly polish the area with a soft cloth. The paint is considered hard-dry when any mark left by the finger is completely removed by polishing."
Next we can look to Mark Gottsgen, author of “The Painter’s Handbook,” and founder of AMIEN, an online source of factual information about artists’ materials and practices:
"No artist wants to wait months to begin a painting, or to apply the final coat of varnish once it’s finished. The truth is, drying rates vary greatly based on the thickness of a painting and on the temperature and humidity levels they are exposed to.
In regards to a finished oil painting, “there’s no hard and fast rule because some paintings are thick and some are thin, so some will dry slower and some will dry faster,” said Gottsegen. “The fingernail test is a really good test. You take your fingernail and you press it on the paint and if your fingernail can leave a mark, the paint is not dry enough to varnish. If it doesn’t leave a mark, like a dent, then you can varnish it. I’ve had oil paintings dry in a month because I don’t paint that thickly but then I have friends who paint with lots of impasto and they have to wait a year if they want to varnish.”
So the bottom line here is not guessing at a Goldilocks zone between one month and a year(+), but rather determining a means by which to successfully and consistently assess whether your painting is physically prepared to receive a varnish. The two tests listed above seem to serve many well (I’ve done the latter quite a bit.)
Hopefully this will make the issue a little clearer for all you painters out there!