Scratch, brown and corner beads.
What is meant by scratch and brown when preparing a wall for plastering. Also is the use of a corner bead common practice when plastering corners. How well does plaster hold if a corner bead is not used?
A scratch coat is the first coat in plastering. For example, it is impossible to put on 3/4" of mortar at a time, so a scratch coat is put on the metal lath and allowed to harden and partially dry before filling in the wall. A scratch coat is used for other applications in plaster, for example, the white coat or the finish coat. The white coat is put on in two steps, a scratch coat and a double up coat.
The brown coat is when the basecoat is doubled up, that is filled in after the scratch coat is filled in. The brown coat is when the wall is filled up and straightened with a rod. Putting mortar on the wall is sometimes called scratching on mortar.
I rarely use corner beads on my jobs. I only use corner beads on newer inside plaster like veneer plaster, and only use corner beads on outside stucco when I need to match the old work, like government work.
There are no corner beads on this historic 1880's fireplace. The outside corners, or arrises, are sharp straight and neat. This was done by using wood strips.