If a patch extends around an external corner, nail a batten with a smooth, straight edge to one side of the corner so that its edge is level with the existing plaster on the other side. Plaster up to the corner so that the new plaster is flush with the edge of the batten.

A: If a patch has to extend round a corner, nail a thin piece of batten to one side of the corner and plaster up to it.

B: When the plaster is dry, remove the batten and pin it to the replastered edge. You are then ready to replaster the other side.