Originally Posted by Richard Bingham:
...Marcus, the principle is correct, but you have it reversed. As paint films continue to dry and give up their volatiles, they shrink They do NOT "grow"...

While I wouldn't refer to it as growing, from what I've read drying oils do expand, possibly as much as 15-25% as they absorb oxygen, and then will gradually lose volume as they release their volatile content over time. This can lead to cracking issues on a top layer surface which itself has already dried. The volatiles which may include acids and alkenes can also stay present in the paint film and react with some types of pigments, which may lead to discoloring with any top layers as well.

Richard, I found this out the hard way, but not consistently and not with Damar. I've used Gamvar with cold wax as the directions said. One came out beautiful. The other one was fine when I applied it, but later attracted all sorts of dust. It had darker tones, so maybe that was part of the problem - that the dust showed up worse?

I have one painting I will have to varnish in late March or so. It's got a lot of dark tones to it, and I am guilty of having used too much retouch varnish (Winsor-Newton). I'm concerned that Gamvar over retouch may not spread evenly - had this problem once before in a similar situation. Do you have any recommendations?

Try this experiment. Put just enough linseed oil in a palette cup to cover the bottom of the cup, and "pull up" visibly on the sides (that would be about 1/16" "deep"). Leave it in a warm, well-ventilated place (plenty of oxygen). At what point do you note expansion of the volume of oil by as much as 10%? What is the nature of the surface of the dry oil film as soon as the puddle of oil "skins over" ? Is it bloated, like a little bubble because of the volume of oxygen being absorbed, or does it resemble a raisin, shrinking in the sun from the loss of its liquid components through evaporation?

When linseed oil is heated to approx 475F in a vacuum in making stand oil, there is a slight noticeable increase in volume, as the purpose is to partially polymerize the oil, which requires the addition of oxygen, but that's not a dry film state.

Richard, I couldn't find Frederick Taubes' "Studio Secrets" at my library but they did have a couple other of his books, both on oil painting, and I put them on hold along with Ralph Mayer's book.

Jennifer, try Abe Books online for books out of print. I was able to "score" a couple of copies of "Studio Secrets" for my students very reasonably a few months ago!

At the risk of an omelet on my Buick ... You are actually both right. In the beginning, the drying paint film gains weight as oxidation means absorption of oxygen molecules, then as the chemical reactions release gases, the paint looses mass.

I am not a chemist but that is the way I learned it.

Ah! "The Rest of the Story". True enough. Oxygen is absorbed, resulting in the polymerization of paint vehicles. Though Mr. Beck offers the disclaimer, I'm confident we may rely on the fact he presents, i.e., absorption of oxygen results in a measurable gain in weight in a drying layer of paint.

The question, however, was not whether the paint gains weight as it takes on oxygen in drying, but whether it gains volume enough through the absorbption of oxygen to cause cracks to appear in a paint film. His subsequent observation, that the paint loses mass with continued "drying" seems to preclude that a surface "skin" of dry paint would be brittle enough to crack at such point as the wet paint is readily absorbing ambient oxygen, if that corresponds to an increase in physical volume.

I am not a chemist either, but I know what I have repeatedly observed in coatings of paint, and so gently refer you to the experiment I offered in a previous post.

Realizing some may find this nit-picking over detail a bit onerous (frankly, I'm annoyed at myself for pursuing it) I appeal to readers who do have a background in paint chemistry for their input.

Mr. Beck, your Buick is safe from me. You are a helluva fine painter!

Do you sell your paintings with the retouch varnish on them? If so, then what do you communicate to your clients about the need for a final varnish in the coming months?

As an Artist I find this subject to be most elusive :) it seems so anyway .... there are tutorials and or articles on every step of the life of a painting from start to finish. But what happens when its finished ..... I mean if its a portrait painting ... your client is going to want delivery of the painting sooner than later not leaving time for a final varnish .... so how do you all deal with that?

Mark

Hello there,

If I won't be able to apply the final varnish. I wait as much as I can to apply that before shipping the painting, something like 2 months. I have not had a bad experience so far.
Otherwise, I tell the client where he will find a professional who can apply the final touch.

Thank you ... I have to say you are the first Artist that shared more than just generalities .... this helps and its much appreciated :)