Roger & ILia/Ian,

Thanks for your replies. Since I'm financially liable for whatever fix I come up with, I'm going to try to leave the Durarock in place. Maybe I will kick myself for this later, but right now there are two options I want to explore.

1) Durarock coated with Plaster Weld with sharp sand mixed in as a mechanical binder, then lime render and finish coats.
2) Durarock coated with a thinset mortar scratch coat, then lime render and finish coats.

As I was demolishing the base coat today, I found that the lime plaster had adhered quite well to areas of a mortar admix that I'd used to tape the joints between the boards. That observation is the origin of the thinset experiment.

I'm not wild about either of these possible solutions. I'm tending to agree that cement board products are fairly useless. Next time I will start with metal mesh and plywood or greenboard. I'm guessing now that's what I see as the fresco canvases.

I'll let you know how the experiments go.

Cheers,
Kirsten