Convert oil primed panel to true gesso panel?

​Hi Christopher.  I do have a few thoughts on your questions (although I think there are other, more qualified voices for some of your queries – hopefully they’ll chime in too).  However, at the moment, I’m in the midst of teaching a workshop; so when that’s complete, I’ll contribute what I can.  Yes, I love the nerdiness and deep dive of this forum too.  Koo Schadler

Hi Christopher. Koo will give a very good exhaustive answer but since that will be delayed at least for a short while I will mention the following:
The fast answer is NO. There will be no assurance of adhesion applying thick true gesso layers over an oil ground, even if it is very lean. There are possibilities of isolated aqueous strokes adhering to very lean oil layers or even very thin aqueous passages, but globally and in the thickness required for a true gesso ground – no.
I will respond to your other queries within the questions and inred.
1.     Can one even paint with gouache/water tempera paint on an oil primed surface, albeit 20 years cured? It seems to violate “lean to fat” rules…
See my caveat above and even then this is far from best practice as evidenced by countless 20th-century experimenters.
2.     Can I “convert” the oil primed surface to true gesso? 
See above, No.
a.     Can I apply an intermediate layer of RSG then several layers of true gesso? If so would increasing the “tooth” by sanding with 220 grit or so help? Thoughts?
No, but of course, sanding would help. No matter what, make sure that the oil ground is not pigmented with lead white if you are going to do any sanding.

b.     Or perhaps shellac? With whatever intermediate steps you think appropriate? Is shellac an ok ground for a true gesso surface? With RSG between?  etc etc
There are techniques for doing this in decorative painting where an oil layer is isolated with shellac and then water borne paints are applied. This is almost never a contiguous and thick layer. I would not trust a ground atop such a stratigraphy.
c.     Is there some other way to “convert” the panels and retain long term stability?
Not really.
3.     Does anybody want to buy 9 beautiful oil primed surfaces?     
Really you should just same them for future oil painting.

​Hi Christopher,
Now that Brian’s replied, I’m inspired to do the same –  and, anyhow, when I’m talking egg tempera all day in the classroom it’s hard to turn off the spigot.
I’ll stick primarily with egg tempera, since that’s what I know best. 
If you can get ET to flow and behave atop a ground (whether that ground is animal glue, synthetic polymer or oil-based) it will likely adhere, short term, to that ground (and, as you note, a light sanding is often enough to get ET to flow and seemingly adhere to many surfaces).  The question is, how well does ET adhere long term?  There is some disagreement on this issue….however, my theoretical understanding of the medium + direct experience tell me that ET adheres reliably long-term to a ground that has all three of the following characteristics: 
1. The surface of the ground is microscopically rough, irregular.  The roughness need not be visible to the naked eye; it can come from a light sanding (i.e. even an extra fine, 360+ grit can open up the surface enough; a rough sandpaper that leaves obvious scratch lines isn’t necessary).  
An additional, even better way to get a microscopically irregular surface is if the ground has a very high solid content (as is characteristic of most grounds), so that solid particles (chalk, gypsum) protrude above the surface of the ground (again, on a microscopic level) – this roughness gives something for the paint to grab onto.
2. The ground is very porous and absorbent.  In addition to a rough surface, there should be porosity within the ground – places for the paint to flow into.  This allows for paint and ground to further mechanically intermesh and bind together – like Velcro.  
3. The binder is receptive to water (doesn’t repel water).  I believe, ideally, the binder in an ET ground should be receptive to water – this allows paint and ground to more deeply intermesh with one another.   
Animal glues remain forever soluble/receptive to water (in fact, they love water!).   Synthetic polymer and oil binders cure and become water repelling (there’s a reason rain gear is made from oiled fabrics or plastic).  Not only does the water repelling nature of oil and acrylic/vinyl binders make it harder to control a water-based paint (the water content in the paint stays closer to the surface & thus is more apt to redissolve paint layers); I also believe a water-repelling binder slightly decreases the mechanical adhesion of the paint.   This is based on my experience of painting in ET, as well as seeing a few instances of delamination of ET applied to oil and synthetic polymer-based surfaces.
In fact, an experienced ET conservator has seen the following: A finished ET painting fully polymerized (so that the yolk binder was no longer water soluble); the painter decided to add additional ET paint layers to the cured surface; those additional layers later peeled off.   There may be other reasons (i.e. perhaps the painting accumulated dust or grime as it sat around and the painter didn’t clean it before applying fresh paint) – still, it’s a cautionary tale. It’s why I recommend, if one wants to rework an older, polymerized ET painting, to first dust/clean the surface, then lightly sand to open it up, then apply a much thinned layer of egg yolk medium (a nourishing layer) before applying fresh layers of ET (still, even with all that prep adhesion may not be quite as good as atop fresh, uncured ET layers). 
Brian and I have discussed the above and are in agreement, I believe.  However, you can find experienced voices who say only the first two characteristics (roughness and porosity) are needed for long term adhesion of ET to a ground, and that the last characteristic, a water-soluble binder, isn’t necessary.   I disagree, respectfully; I’m a big fan of an animal glue + chalk/gypsum ground for ET.

One more clarification… As oil paint experts on this forum have pointed out, another (perhaps more accurate) way to think about “fat over lean” is that you want more flexible layers over less flexible layers.   Whichever way you think of it, I wouldn’t say the issue with ET atop an oil ground is primarily a fat over lean or flexible or less flexible, issue – it’s really more about adhesion. 
Of course, long term adhesion is dependent on lots of factors.  ET on a synthetic polymer or oil ground may last for a long time if the work lives under good conditions; conversely, you can follow all best practices and a painting delaminates because it runs into challenging conditions.  Nothing’s guaranteed either way.  Still, in my opinion I would not paint ET atop an oil ground (even if it is a high PVC, rough/porous ground – which you’re not entirely sure of).   Nor would I apply gouache or animal glue for more or less the reasons given above.  This, unfortunately, nixes applying true gesso on top. 
)-:
However, I might (particularly if you could figure out if it actually is an oil ground, not paint – since a ground has more roughness and porosity) consider working with tempera grassa on top; even a water-based, egg-oil emulsion may adhere decently.   Just  make sure the surface is free of dust/grime and roughened/opened up.  

Then again, there’s always Etsy.  

Good luck.
Koo

​​​Hello Again,
Well, after replying I realized that your post states “gouache/tempera…(Not egg tempera).​”  Whoops – appologies for not reading it more carefully.   If you can clairfy the binder in your ‘tempera’ then the appropriate person can chime in.  I still think goauche (with its non-polymerizing gum arabic binder that remains forever soluble) requires the absorbency/porosity of paper or an animal glue + chalk/gypsum ground to adhere well; and, ideally, I wouldn’t apply animal glue or traditional gesso​ atop an oil primer.  Layering shellac inbetween would increase adhesion in the short term, but in the long term that sort of complicated layering invites problems (and in complicated systems probelms are hard both to diagnose and solve). 

Koo