Key

A key is what makes the plaster stick to the background so all backgrounds must have a good key.
There are 3 types of key:

Mechanical key: This is where the surface is keyed before by using a special type of keyed brick or lightweight block.

Spatter Dash Coat: This is a coat made from made of cement and sand. It is applied either by a machine or a dashing trowel.

Hacking the surface: This is where you physically start hacking at the background to create a key. This can be done with a toothed hammer or a scabbler.