Painting over old canvas
 

I know it is considered not kosher to paint over old canvas for various reasons: paint chemistry, bonding, lack of bonding, etc..

But how does that square with guys like W. Homer, who once spent up to 12 years finishing a canvas? Surely, at some point in that process, his original canvas had reached the point where paint chemistry had changed, etc., etc. In short, it had become an old canvas.

I'm not arguing FOR painting on old, previously used canvas, but it is done all the time in one way or another. I wonder how these "long term Homer-like" projects weather the storm in the long term?

I, myself, find that every now and then, I change my mind, and change a section of a painting, which, in effect, is painting fresh paint atop a previously "used" portion of a canvas.

Just some thoughts. I'd appreciate hearing yours.

The only caution I've heard is about painting over a previously completed canvas when it would be hard to follow the rule of fat over lean. (Presumably the upper layers in the old painting will be fairly fat and you'd want to start with lean layers in your new painting.)

Is a fat layer that has completely dried still fat?

Yes. The oils in our paint don't evaporate away when the paint dries, they chemically oxidize to become dry to the touch. They're still there.