This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Preliminary Examination. Although the carpenter's specifications require that all surfaces be properly prepared for lath and plaster, and that suitable plaster grounds, such as shown at e, Fig. 104, be affixed to the masonry and framework for the future attachment of the finished joiner work, it is well for the plasterer to test the alinement of the walls and ceilings, or he may find that considerable "building-up" is necessary when he comes to apply the second, or floated, coat. Where this is necessary, the lathing should be stripped off, and the surfaces brought to a plane by the use of furring strips.
In the case of dished, or concave, surfaces on stone or brick walls, the deficiency should be made good with cement mortar before the first coat of plaster is applied; this course will insure a more durable wall, as there is little value in a mass of lime mortar unless it is applied in thin layers and thoroughly compacted.
Particular attention should be given to the spacing of the lathing; although a simple matter in theory, it will be found that in practice, unless much care has been taken in spacing the strips, that many narrow, insignificant slivers have been used for closers; these should be discarded and the panel stripped and relathed, otherwise much of the strength of the wall is lost. For three-coat work, the grade in general demand, the plaster grounds on brick or stone walls, that are to be coated on the solid wall, should be 5/8 inch in thickness, and those for lathed surfaces 7/8 inch. At no point should the surface of the solid wall or that of the lathing approach the face of the grounds nearer than the regulation thickness for the plaster, which will be about 5/8 inch. On stone or brick walls, the face of the masonry is more or less irregular, and grounds less than 5/8 inch in thickness are of little value for attachment of the finished woodwork.
Names Of Coats. Plaster is usually applied in three coats, although in some localities two is the customary number, the first coat being straightened and smoothed carefully to receive the finishing coat. This, however, is done in the cheaper kinds of plastering only. The first layer applied is called the scratch coat; the second, the brown, or floated, coat; and the third, the skim, white, or finishing coat. On brickwork and stonework, and also on terra-cotta partitions, etc., the scratch coat is usually omitted.
Scratch Coat. The method of applying this coat is illustrated in Fig. 104. The coarse stuff is taken in batches from the souring pile, tempered with clean water to the proper degree of firmness, shoveled into hods, carried to the rooms and deposited on the mortar boards, as shown at f, ready for application by the plasterers. A quantity of mortar is placed on the wood hand board, or hawk, g, by means of the trowel h; slices of the mortar are then cut from the hawk and spread firmly and evenly over the surface of the lathing or naked wall surface, as the case may be. The mortar should have a dough-like consistency, should be tough and hold well together, and should be soft enough to be readily pressed in between the laths, so that it will bulge out behind and form the clinch, or key. The thickness of the layer should be a full \ inch, so that when set it will furnish a rigid surface to work over; in cheap work, it is often only a skim coat, and it is not unusual to see the rough-sawn grain of the lath protruding through its thickness. After the coat has somewhat hardened - which requires from 2 to 4 days after application - it is scratched over diagonally by wooden comb-like blades i; from this fact the first layer is often called the scratch coat. The scratches, or grooves, thus formed in the mortar fulfil the same function as the spaces between the lath - to allow a good key for the subsequent layers. The first coat should be nearly dry before putting on the next; if too dry, the surface should be slightly dampened with a sprinkler or brush, on applying the second coat If the walls are partly masonry and partly wood, the first coat is only applied to the lathing - unless the stone or brick walls are furred and lathed also. When the scratch coat is dry, the second coat is spread over this and the masonry. If plastering is applied directly to brick or stone, the joints in the latter should be raked out, so as to form a better clinch for the mortar, and the walls should be free from dust, and be slightly dampened before putting on the mortar.