Prep a plastered block wall for tiling in shower

[Xen]

What's the best way to prep a plastered breezeblock wall for tiling in a shower enclosure?

One of the walls of this enclosure will be a new false wall for various reasons, but the other is part of a large external cavity wall which is already plastered, though I've scraped off a lot of the loose finishing plaster so its a bit of a mix of finishing plaster and hardwall. I don't really want to dot n dab over the top as it will be out of level with the rest of the wall, so I'd have to do the whole lot. Can I just paintable membrane it, or is there a better solution?

 

[flameport]

What's the best way to prep a plastered breezeblock wall for tiling in a shower enclosure?

Get rid of the plaster and start again.

 

[Xen]

What should I use to build back out to level? Presumably it's the gypsum that's the issue? Could I scrape back to hardwall, prime and tank it before tiling, or is that going to still cause problems?