Oil Paint on Shellac

First, shellac would not be my first choice for a size for wooden panels but it should be perfectly serviceable. It does benefit from being delivered in a non-aqueous solvent. This means that the wood will not have its grain raised or issues of warping caused by the application of a shellac size. A size should never be applied so thickly as to create a discrete layer and certainly not a glossy surface. Any size applied too thickly will cause problems with adhesion (actually this is less of an issue when painting acrylic dispersion colors on acrylic dispersion grounds since the acrylic dispersion binder is such a good adhesive as compared to oil paint) . See our section on sizes in our “Resources” section for general issues about sizing.
If the shellac is applied in a dilute enough state that is cuts and evens out the absorbency of the panel but does not create a discrete layer which would eliminate mechanical adhesion of the ground and paint layers, this should be a trustworthy panel. Whether these specific panels are “acceptable and durable” would relate to the above as well as what type of panels they are sealing with shellac. No size is likely to make a junk panel acceptable.
There is actually a long history of the use of shellac as a preliminary application before oil paint in the house painting trade. I seem to remember reading a house painting manual from the early 20th century where it said to, “give the wall a good drink of shellac before painting in oil paint,” or something to that effect. I just checked and R. Mayer also mentions this reference.
Now, the above is specifically about the use of such a size under an oil ground and oil paint. I can see no reason to use a shellac-sized wooden panel for subsequent acrylic dispersion grounds and paint. In that case, it would make more sense to size the panel with acrylic dispersion medium or perhaps PVA size. Animal glue, if one choses to use it despite its reactivity to the environment, should only be used under oil and alkyd layers.

There should be no problem in terms of paint adhesion/etc for you to paint directly on the shellac sized panel. The only issue would be the color of the panel. Depending on how dark the wood is, this would be visually similar to painting on a mid-toned or even dark colored ground. We know from examining old oil paintings that oil paint can becomes more transparent overtime. This is mostly, but not completely, the result of a chemical change in lead white and even zinc white paints. Thinly painted lighter or even mid-valued paints can become darker because of this increased transparency. This sometimes resulted in an exaggerated contrast effect where the darks became slightly darker, the midtones disappeared, and the thickly applied highlights remained light. Othertimes, it caused an overall lowing of the value and intensity of the painting.
We are not sure if paintings made with titanium white will exhibit this increased transparency of time but it is unlikely to do so to the same degree. We now know that oil painters should avoid the use of zinc white for a number of reason. I am also definitely not suggesting that oil painters should avoid lead white. It is really the best oil paint that we have in terms of preservation. Additionally, there are plenty of paintings from the 17th-21th C. where that artist left portions of their wooden panel exposed for a specific effect. We just do not know the degree to which these have shifted overtime. What I am suggesting is that a painting executed on a darker color (like could be the case with shellac sized wood) should not be painted too thin if the artist wants to avoid possible changes overtime.