How thin can a bonding coat be?

[Cockney]

View attachment 35078

I Ain’t opening none of your videos ever again!!! Lol.

 

[malc]

The whole flat has grey undercoat on brick all, they are definitely not sand cement, several builder says it's bonding from 50 years ago. I said bonding+skim for tiling because it's the only combination I have done before for tiling so far.

what you have on your walls is Carlite browning. not bonding . hack off and replaster in thistle universal one coat plaster.