As most of the painters here know, an ideal varnishing schedule is often governed by many factors. At present, most try to abide by the time frame that is put forward in Natural Pigment founder George O’Hanlon’s paper on varnishes. He writes, “A: 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.” However, some have put forward some arguments to challenge this long-standing 6-12 month recommendation. Once such argument comes from professional artist and chemist Chuck Mauldin. In his paper, “When to Varnish: How long should an oil painting dry before varnishing?” Mr. Mauldin puts forward his evidence and asks the reader to review, research and decide for yourself. (Excerpt and link to the PDF below.)
Excerpt: "How long should an oil painting dry before varnishing? Six to twelve months? Sounds like a prison sentence, and it is when imposed on a new born oil painting! But that’s the range of time recommended on most varnish labels for allowing an oil painting to dry before varnishing. However, faced with deadlines of competitions, gallery shows, commissions, and heightened by the excitement of getting that new painting to market, artists often choose to compromise on this recommendation. We varnish early, or not at all, or maybe apply retouch varnish as a stopgap, perhaps with some sense of guilt, concern, or confusion regarding this final step in preparing a painting for longevity. It is amazing that, with all of the excellent information available about painting, rarely is the topic of when to varnish even discussed. Such an important topic, yet little is truly established.
One thing is clear. Everyone wants their paint layer to be properly cured to a strong, durable, insoluble film. That is expected and will not be compromised. Most also want to apply a varnish to their oil painting in order to (1) protect it from dirt and grime with a removable outer layer and (2) provide an attractive, uniform finish. It’s the final step in creating a professional looking, built-to-last work of art. Unfortunately, these dual desires of properly cured and varnished can seem hard to satisfy when a six month waiting period is enforced.
The frustration is further aggravated by the lack of reasons or explanations as to why such prolonged drying is needed. Nothing definitive is stated technically, much less supported by scientific evidence, in reference books, how-to books, magazine articles, or the web. There is an assumption that the varnish can adversely affect the curing process in some way. For example, some have claimed that varnish may block the oxygen needed to harden the paint. Others have warned of premature cracking. However, a full explanation and supporting data are strangely absent.
Fifteen years ago, I embarked on an effort to answer the question of “How long should an oil painting dry before varnishing?” Part of this quest involved reviewing some of the available technical literature concerning the curing and aging of oil paint. Chapter 2 is devoted to making an artist-friendly, simplified summary of this complex, voluminous information. With this as crucial background, a variety of simple experiments are described in Chapter 3. Most importantly, the implications of my experiments for varnishing are discussed in Chapter 4.
And the answer? At least three weeks (of course, that depends on many factors, like the presence of driers, mediums, and the thickness of the paint, to name a few). No reason to wait six to twelve months was found. Bottom line: if it’s dry enough to varnish, it’s dry enough to varnish! That’s my opinion, not my recommendation. The latter implies liability. So, please read my evidence and decide for yourself." -Chuck Mauldin, When to Varnish?
LINK to PDF: https://userfiles.faso.us/40607/8721.pdf