GFJ,
I think your project is very doable. If your masonry grid is similar to diamond steel mesh, or stucco lath and your underlayment is properly treated there are sealers that can be used to coat the plywood so it is more water repellent, it should work great. Or you could attach wodnerboard or durock (both concrete/cement boards) to the plywood You may need to apply a undercoating/scratch coat to the grid first before applying your tufa in faux rock form. You can form and sculpt tufa freehand. You would be able to work in small batches rather than trying the whole 7x7 at one time. If you mark out your grid and kind of bounce around it will look more natural.
Try a small sample on a piece of plywood with mesh to get an idea. Dena

Stucco is water proof.
If your tufa isn't your building will rot from the inside...quickly as well.
No amount of water repellent in the world will save you.:)

Dena, do you think the hypertufa (& I'm talking with peat, here) would really be a good material in NJ? They get some good freeze/thaw there, I think.

Now, if you meant concrete, even with a good bit of perlite in it, but no peat, wouldn't that work better, as there's much less chance of water movement through the material?

Sue

I would scratch coat first.
Find a nice soft garden spot, and several big and small stones with a nice shape and texture. Press the stones into the damp ground about 1.5 inches then pour a super light weight tufa made with perlite and a pinch of sand into the depressions.

Use Steve's spray paint method to color your faux stones then glue them to the wall.:)
Spray finished with a satin finish water based concrete sealer.

I have one of those white plastic calf houses that I want to cover in tufa and have been thinking about the different ways for doing this. I think it would make a funky garden shed. I wonder if I could first cover it with burlap soaked in concrete slurry first, then tufa over that. Maybe with the steel mesh over the burlap? I want it to be easy, but it might not be.
Jo

Sue,
I usually forget to look at where the person is, so will defer to someone with experience in that area. You are right the mix should fit the area where the person lives.
I do not use bonding agent or fortifier in my 'tufa mix' very often, but if I wanted less water to transfer through that should be added. I have found that with bonding/fortifier I must come back to 'rough up or change' the surface muche more quickly than with other methods.
And yes a person could make natural earth molds, or damp sand molds to make the rock to face something.

"Standard Practice" for applying stucco involves applying a layer of waterproofing over all wood to prevent rot. Tarpaper is cheap, adequate and what most plasterer's use. Then attach thin wooden lath strips horizontally over that and apply stucco cloth (metal lath) to the wooden strips. This allows you to press and work the base coat through the metal cloth for a solid base. From there, you can finish coat as desired and be confident that it will "breathe" properly and stay put for many years to come.

Tango88 is correct. Concrete "sweats" -- if there's a way around that, I haven't heard of it. Even the floor of an enclosed garage will sweat (which is why you don't leave bags of cement or books sitting directly on the floor). And that sweat effect will cause the wood to rot if you don't make a point of dealing with it first.

Sue

Dear All-

Thanks for the input. I think I have some valuable advise. I will post pictures when we finish the project.

I learned the requirements are: a good Vapor barrier, slats, concrete wire mesh, grout,hypertufa with no peat (maybe the fibre mesh- where do you get it?), seal the hypertufa once cured. I think the fact that I do live in an area where freezing happens will require me to piece this facade so no large parts that can crack- using the cement grout as foundation and adhesion will be necessary. Thanks