Egg tempera on watercolour/illustraion board

Traditional gesso serves a few different roles for egg temepra:
– It absorbs the water content in egg tempera paint, which gives an artist more control and discourages lifting (since water is the diluent for tempera; if water stays on the surface, it tends to repeatedly dissolve and lift paint). Having water whisked away into gesso counters lifting.
– It encourages mechanical adhesion; the paint settles into porous, absorbent gesso to interlock with it, and adhere more securely.
– It provides “storage” for the “mobile lipids” in egg oil; i.e. the percentage of the yolk binder that never polymerizes (cures), but can move around in the paint film.  Without this storage, there’s a greater chance of efflorescence on the surface (fatty acid migration) which can create a whitish fuzz (it can be brushed away, but still isn’t desirable).
The more layers of gesso applied, the more you get the above benefits.
All “gessoes” (grounds) have a percentage of solid pigments (i.e. chalk) in them, to make them porous, absorbent, & with a rough surface, to encourage adhesion between ground and paint layers.   Traditional chalk and glue gesso has an especially high pigment load (which is what makes it so wonderfully absorbent and well suited to egg tempera).  The downside is that pigments are hard solids that don’t bend – the more pigments in a ground, the less flexible a ground, and the more important to apply that ground to a rigid support.
Egg tempera also has a very high pigment load, so it too isn’t very flexible.  It lasts best on a rigid support; or, if on a less rigid support, applied thinly, in just a few layers.
Tubed egg temperas have a percentage of drying oil in them; this makes them a bit more flexible.
With that intro, onto your questions…   
1.  Has anyone tried using traditional egg tempera or tubed egg tempera on watercolour or illustration board?  Can you work directly on the surface?
Both homemade and tubed egg tempera can be applied directly to watercolor and illustration board (and paper, for that matter).  However be aware that those surfaces aren’t as absorbent as true gesso, so if you work with watery paint (petit lac, glazing/scumbles) or build up lots of layers (which accumulate water) you may experience lifting more readily.   Also, depending on the rigidity of the board, those supports may warp if you inject lots of water, and a flexed support can lead to hairline cracks in egg tempera paint as it ages.  
Both traditional gesso and egg tempera paint, due to their high pigment load, do best atop rigid, nonmoving supports.

Don’t use a so-called “vellum” board, as water-based paints tend to resist/crawl on very smooth surfaces.  
2.  If so, do you need to prime the surface with traditional gesso? You don’t have to – but certainly gesso helps egg tempera paint behave better.  However, for the reasons given above, I don’t recommend applying lots of layers of traditional gesso to paper boards – they’re just not rigid enough.  One, maybe two, thin layers of traditional gesso are okay; however a single layer minimizes the benefits of traditional gesso (which are best obtained with multiple layers).
In short, for best durability I recommend no more than a few, thin layers of egg tempera on paperboard.  You can paint directly on a paperboard or apply 1-2 very thin layers of gesso.  
If you want to build up a more complex, multi-layered egg tempera, or work with lots of water in the paint, you should use traditional gesso (something like 8-10 coats or more) on a truly solid support (engineered wood, a properly prepared solid wood panel, or perhaps an aluminum panel, if you first coat it with fabric so the gesso has something to grab on – although the jury is out on aluminum for ET and frankly I have my doubts…).
Of course, you are also free to apply as many layers of traditional gesso and egg tempera as you like to paper boards; it would likely work in the short term, maybe work in the long run – but you run a much greater risk of problems like lifting, cracking, delamination, and efflorescence.

You and I previously communciated via email, so I just want to say thanks for sending your quesiton onto MITRA.

​Koo Schadler

I am away from the internet until August 1st