Delamination

Sorry for the late (and incomplete) reply. In my experience, there is a wide spectrum of absorbency among brands of acrylic dispersion painting ground (gesso). Some are more resistant and “fast” while others are so thirsty, the initial paint layer can look dry and waxy. I don’t think I’ve ever used an acrylic ground that was less absorbent than a dry oil painting, though.

Hi Koo
I just returned from a conference in Belgium and finished grading so I am only now getting to your question. I will respond to each point within each section of your question and in red.
Hi Brian,
In a reply to a recent post you said, “…works painted over earlier compositions are much more likely to develop delamination issues over time.” This raised questions for me.
1.  Why is this so? Is it because newly formed oil paint films don’t crosslink with dried paint underneath?  How much less absorbent is a dried oil paint film versus acrylic gesso ground?  And were you referring to oil paint only, or other mediums?
It would be an oversimplification to make a definitive statement that covers all situations, but I will try to make some general statements. I do not think that it is an issue of the newer paint not “cross-linking” to the older layers but another group of possible issues. First is the diminishment of absorbency and tooth. This is inevitable. Additionally, if the underlying painting was taken to a high level of completion the artist may also be have added additional medium in their later layers exacerbating the points above. If there was a substantial period of time between the first and second composition, there could also be a certain amount of surface grime that could slightly compromise adhesion. It is also generally true that the more layers used to execute a painting, the more likely a problem can arise. Finally, there does appear to be some closing off of the surface of a painting that occurs after the paint has dried for a while. I have never seen this completely quantified but people have long spoken of the presence of an extremely thin layer of oil that sits above the surface of the pigmented layer. Perhaps due to a slight sinking of the pigments. This may contribute to a closing off of the surface. Truthfully, I have my doubts about this and its effect. I have, however, in my years conserving old paintings. I have seen many examples of paintings that were overpainted in oil by an unscrupulous restorer. Sometimes a very cautious and highly trained conservator can this cleave this overpaint cleanly off of the original paint without damage to the lower imagery. This does suggest the lack of true adhesion between the original and the later application of paint.
I am not sure that I would make a definitive pronouncement, but generally, dried oil paint (that has not been overly thinned) would be less absorbent/toothy than a properly formulated acrylic dispersion ground. Having written that, a good lean lead white oil ground can retain a good deal of absorbency.. 
The above is somewhat true of all mediums but there are very different degrees. It is most pronounced with oil paint. It is much less or almost inconsequential in acrylic dispersion painting where the medium is such a great adhesive. Egg tempera probably lies between but my own experience with tempera would put it closer to oil in the sense that the absorption greatly diminishes as layer upon layer of paint is added. It is very difficult to apply broad washes early on without getting overlaps while this is relatively easy after many layers of paint have been applied.
2.  Would you say the same is true for egg tempera; that delamination is more likely to occur in works painted over earlier compositions?
Probably, but it would depend on how many layers of tempera were used in the original composition and how dry (absorbent) or greasy (unabsorbent) the surface is. There are just too many variables to make a definitive statement.
3. The case is often made that dispersive adhesion is primary; however in my experience with egg tempera mechanical adhesion seems equally important.  When I’ve continued painting on aged egg temperas (from a few weeks to over a year old; i.e. partially or fully polymerized surfaces) the paint is more difficult to work with; much more prone to lifting if I do things like sponge on watery paint, or lightly sand or polish as I develop layers.  This seems to indicate less than ideal adhesion between old and new egg tempera paint layers; and that they “are much more likely to develop delamination issues over time”.  And yet I’ve also been told (by well-informed people) that it’s fine to paint atop old, polymerized egg tempera with fresh tempera (as long as the surface is clean).   Your thoughts?  .
You need either chemical or mechanical adhesion, preferably both. There may be a similar skin that forms on the surface of the egg tempera as it oxidizes over time. There are really a ton of oily components in egg yolk. This would be a great research project. I can’t say with authority but you, and my experience suggests that there is a diminishment of adhesion. I suspect that most of it is the loss of mechanical tooth/absorbency.
4.  To address the less than ideal working properties of fresh tempera paint applied to a polymerized surface, I do three things to the surface: 1. Wipe off dust, 2. Do a gentle sanding with a 1500-grit sanding pad, to open the surface, 3. Apply a very thin nourishing layer of egg yolk medium (1 part yolk to maybe 8 parts water).  I’m actually not quite sure why I do #3, except that it seems to help the paint grip and behave better – but I also wonder if the nourishing layer is more detrimental (i.e. contributes toward fatty acid migration, or even delamination) than helpful.  Again, your thoughts?
I would probably err on slightly roughing up the surface over adding egg water. The later probably does help but adding a bunch of the adhesive (binder) which by the very nature of its stickiness will adhere to the lower paint and its freshness allows adhesion to the upper layers. It does, however add a slightly slick interlayer, and more importantly, contributes a superabundance of free fatty acids, which will possibly/probably effloresce from the surface in the future. We have seen this on many 20th century egg tempera paintings.

Brian

Thanks as always!
Koo