The age-old advice to wait at least six months before varnishing oil paintings is a good practice but one that many artists resist. And it is understandable why because when a painting is completed, it often needs to be delivered immediately for exhibit or into the customer’s hands.
The Big Sleep before Varnishing
So, artists often tout the recommendation to wait until the painting is “touch dry.” For some, this may be a sufficient amount of time. This recommendation, however, assumes that all paintings are created under the same conditions. Of course, we know this is never the case since one painter paints on absorbent substrates, another on non-absorbent surfaces, another will use lead white, others will use slower-drying titanium white, many artists paint with impastos, and others with thin applications of paint.
A few manufacturers claim that their varnish can be applied much earlier, such as when the painting is “touch dry,” so many artists grab this idea and run with it thinking the moment the paint surface feels dry, they have a green light to varnish. However, even these manufacturers are a little more cautious by recommending testing the surface of the painting for sufficiently dry, such as with a “fingernail test.”
Somehow many artists believe there is something special inherent in these varnishes. One blogger writes :
In reality, all varnishes are permeable, so oxygen will diffuse into and through them, allowing the polymerization of oil paint underneath. All varnishes slow the ingress of oxygen into the paint film, thereby delaying the drying process.
Waiting at least six months is still the best practice for many painters. Instead, an artist may use (with due caution) the test described below as a method to determine when the painting is ready to be varnished if she cannot wait the prescribed six months.
The “fingernail test” may be what some manufacturers had in mind for the test method for “Dry Hard Time” described in ASTM D1640 :
Variations of this test may be used but will also give different results. A problem with using a hard and sharp object such as a fingernail (in contrast to a flat thumb) is that it can give false results depending on the pressure exerted on the paint film.
Whereas the test described above may work in your case, always keep in mind that some interpretation of the results is involved, often leading to different conclusions. The safest and best practice is to wait the prescribed amount of time—at least six months. But if you cannot do so, use the test method ASTM D1640, and good luck.
For more information on varnishing, please consult this article: Selecting the Right Varnish for Your Painting
Debra (2017) How To Varnish A Painting: The Easy Way. Red Palette Studio. August 17, 2017. View Online. Last accessed 15 March 2020.