[Juan Martinez]

Hi Rod,

I haven't posted for a while, but used to quite a bit. Hope you don't mind if I add my two cents worth.

Actually, shellac shouldn't be considered as a painting "ground". It may have been used as such from time to time, but people also paint on glass, wood, and metal, too, and none of those are considered "grounds". At any rate, shellac was traditionally used to treat chalk-based gesso, a true ground, to cut its absorbency which can vary from batch to batch and is usually a little too much. However, there are two problems with shellac, one of which you have dealt with already.

First, shellac is prone to embrittlement and darkening. Since you are using a panel, which won't deflect as much as stretched canvas will over time, the brittleness shouldn't be a problem. And, since you're using it underneath the painting, rather than on top as, say, a varnish, its darkening shouldn't be a problem, either.

The second thing, though, is that shellac can form an impervious layer which will not allow a proper mechanical bonding of the oil paint with the gesso, which is what oil paint and gesso are supposed to do. Shellac should be thinned with methyl alcohol before being applied to gesso so that it soaks into it, rather than forming a layer on top. This should cut the absorbency of the gesso somewhat, but still allow the oil to penetrate to a degree. It's a rather vague target, at best.

For sketches and the like, painting on shellac might be alright but you're otherwise compromising the permanence of your work.

As an aside, this is very much the problem found on many of Norman Rockwell's paintings; they are darkening and cracking ...all sorts of things are happening to them. Some of this could be due to the shellac, some of it to the kinds of oils and driers he may have used, and who knows what else. Not all of his paintings are deteriorating, mind you, but many are. I suppose he didn't do the same thing to every painting and, he probably experimented a lot, too. As a successful commercial illustrator, he was concerned with speed and not always with permanence. So, whatever got him to his conclusion the fastest and most efficiently, that would be what he did. As much as I love Norman Rockwell's paintings and his abilities, I wouldn't follow his technical practices. (With respect, Richard).

Hope it helps.

Juan