Drywall – How to salvage this ruined wall so it can hold paint

We have a big wall plastered with clay.

It was originally brown, and we wanted to paint it white or a light pink, using lime paint. Lime paint is said to be an ideal match for clay plaster, and has beautiful moisture-regulating properties.

However, we failed spectacularly. Any paint that we try to apply to the surface comes right off, often taking a part of the original plaster with it. We also used a silicate-based ground coat that is supposed to work in situations exactly like this. (We did not manage to get hold of a caseine-based ground coat where we live.) All to no avail.

The result is a huge mess that seems unsalvageable. Anything we try to put on the wall comes right off in places because it's so porous.

The problem is especially big on the left hand part of the wall (see the photo below) and we're pretty sure there has been water damage to the wall, caused by a leaky roof. On the right hand side the problem is there, too, but much less so. It seems like moisture has ruined the wall.

We really don't know what to do any more and are looking for ideas.

We have considered:

• Scratching off the clay and trying to work with the concrete underneath: will likely not keep, as the concrete is as porous as the clay on top of it.

• Scratching off the clay and trying to build a new layer of plaster: we fear that new layer would come right off, too

• Trying out just normal, white dispersion paint – very likely wouldn't keep

• Building a new wall out of blocks or wood: extremely expensive and an insane amount of effort

• Building a faux wall in front of the ruined one, out of wooden panels, plastic foil, or some other material: we're so desperate we're open to this, but couldn't think of anything that'll resemble a real wall and not cost thousands

• Using epoxy as suggested here or some other super-strong grounding – it might be an option, but we fear it'll simply come right off together with a bit of the plaster underneath. Advice based on experience would be welcome.

• Using normal, white dispersion paint after applying some super-effective grounding – perhaps an option? Does anybody have an idea whether this would work?

The wall is load-bearing and we can't remove it.

We are based on the Canary Islands and may not be able to get our hands on very special materials – but don't let that slow down your imagination for the moment.

Advice would be most welcome!