This is an area where I am just about to resume my experiments. It is a system which has evolved from my experience with plaster and concrete. I am not interested in 100% historical accuracy - I am after a system which works - the panels must be lightweight as possible, safely shippable by U.P.S., stable forever in a light exterior environment, and the final intonaco layer must be the real thing.Where I am at so far. Review elsewhere in this forum for missing details.
1) I cut out pieces of "hyrib" expanded metal lath to the rough sizes I like. On a flat bench, and/or piece of plastic, a mixture of about 1 part white cement, 3 parts clean sharp sand, is trowelled onto the lath, making sure it goes through all the holes of the mesh. If the mixture is to dry and stiff, you can add some lime putty to loosen it up. This layer is usually pigmented with inexpensive (compared to fresco) concrete colors (Davis is a good brand), I like a chocolate brown - so that color is mine, pick you own :p. The thickness is about 3/4" - now make sure the surface of the slabs is "raked" like scratched with a metal comb, quite coarsely, before the slab cures. Now cover the slabs with plastic, keep moist for at least three days, heat is OK, but not dryness. It takes 30 days to fully cure, but you can start painting after three days.
2) I run a loop of stainless steel aircraft cable through holes drilled in the slab with a small masonry bit drill. Clamp the cable with copper ferrules. The tools and supplies are available at a local "rigging" shop or sailboat shop. A special cable cutter and clamp pliers will be very handy
3) When you know you will be painting in a few days, select slab(s) from your pile and soak in water, at least overnight (one of the blow up kiddie pools could be handy). The slabs should be damp but not dripping wet, get some concrete bonder liquid and brush it on full strength to the areas you will be painting. The next layer is a modified intonaco, use a coarse and medium sand about 2 1/2 parts to one part lime putty and I also add white cement powder about 1/2 part. Trowel it on, again leave a rough surface, but not as rough as the previous layer.
4) These curing slabs are different from normal fresco because we can more easily control the cure. If kept away from air, the lime will not cure - the white cement will, just enough to stiffen up the putty. So these slabs you can keep around for at least a day or two, maybe much longer, as long as they are sealed with plastic wrap. When you are ready to paint and have time to watch the slabs, the final layer is a thin 1/8 to 1/4" layer of lime putty and fine white marble (or dolomite) sand approximately one to one mix. Trowel out smooth and you should have enough experience to know when to begin painting.
This is my defintion of portable panels. Diego Rivera's portable panels were 3" thick! I think this system could be used by both the professional artist, cranking out masterpieces on a daily basis, or for the part-timer. It may not be suitable for the first timer - its too hard to explain consistency and troweling and when the surface is ready to suck in pigment and when it is too late.
I realise that this may start more questions - but thats ok. Remember that I am learning too - but I did have some delamination problems with my first experiments. The batch prepared as above has been outside all winter (heavy exterior environment), the surfaces (not water glassed) degraded but these slabs are now strong enough that I can actually walk on them like a bridge.
I also wonder if the surfaces would have held up better if they were left a couple of years to cure, rather than a couple of months. Delamination is caused by different expansion ratios of the materials.
