Preparation of concealed cistern for plaster adhesion

Hi don’t know why you would use lime plaster when there is no need but I would cover ply in 9.5mm plaster board Coat board with strong bonding agent like pr360 then plaster PB

If you are using a wall hung toilet pan, use the ply for the strength and plasterboard over and a plaster finish of your choice,, if not a wall hung cistern no need for the strength of the ply.. tadelaka is fine on a plaster board

Tip with Tadelakt lime plaster, make sure you fix the boards well. Any movement will show at the joints ( even if they a scrim clothed well) Your Tadelakt needs a lime base not pink plaster. Cement board is very thirsty & will require soaking well before the lime plaster application, ( even with the primer on) once the lime had cured then you can apply the Tadelakt finish ( you may find it will craze, but this is worked out with the stones) The reason you don't use pink plaster is because the two compounds are completely different. Pink plaster will not tolerate moisture unlike lime. Hope this helps

Use 9.5 ply then 9.5 plasterboard you then end up with 18 mm finish then you can skim with whatever you want Or put blue grit on ply then skim Whatever is cheapest option.