How thin can a bonding coat be?

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.
 
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