Final varnish

[David Clemons]

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 Bingham]

Originally Posted by David Clemons

. . . from what I've read drying oils do expand, possibly as much as 15-25% as they absorb oxygen . . .

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.