Plasters are applied in successive coats or layers on walls or lathing and gains its name from the number of these coats.
One coat work is the coarsest and cheapest class of plastering, and is limited to inferior buildings, such as outhouses, where merely a rough coating is required to keep out the weather and draughts. This is described as render on brickwork, and lath and lay or lath and plaster one coat on studding.
Two-coat work is often used for factories or warehouses and the less important rooms of residences. The first coat is of coarse stuff finished fair with the darby float and scoured. A thin coat of setting stuff is then laid on, and trowelled and brushed smooth. Two-coat work is described as render and set on walls, and lath, plaster and set, or lath, lay and set on laths.
Three-coat work is usually specified for all good work. It consists, as its name implies, of three layers of material, and is described as render, float and set on walls and lath, plaster, float and set, or lath, lay, float and set, on lathwork. This makes a strong, straight, sanitary coating for walls and ceilings.
The process for three coat work is as follows:
For the first coat a layer of well-haired coarse stuff, about 1 inch thick, is put on with the laying trowel. This is termed "pricking up" in London, and in America "scratch coating". It should be laid on diagonally, each trowelful overlapping the previous one. When on laths the stuff should be plastic enough to be worked through the spaces between the laths to form a key, yet so firm as not to drop off. The surface while still soft is scratched with a lath to give a key for the next coat. In Scotland this part of the process is termed "straightening" and in America "browning", and is performed when the first coat is dry, so as to form a straight surface to receive the finishing coat.
The second or "floating coat", and is 1/4 to 3/8 inches thick. Four operations are involved in laying the second coat, namely, forming the screeds; filling in the spaces between the screeds; scouring the surface; keying the face for finishing.
Wall screeds are plumbed and ceiling screeds leveled. Screeds are narrow strips of plastering, carefully plumbed and leveled, so as to form a guide upon which the floating rule is run, thus securing a perfectly horizontal or vertical surface, or, in the case of circular work, a uniform curve.
The filling in, or flanking, consists of laying the spaces between the screeds with coarse stuff, which is brought flush with the level of the screeds with the floating rule.
The scouring of the floating coat is of great importance, for it consolidates the material, and, besides hardening it, prevents it from cracking. It is done by the plasterer with a hand float that he applies vigorously with a rapid circular motion, at the same time sprinkling the work with water from a stock brush in the other hand. Any small holes or inequalities are filled up as he proceeds. The whole surface should be uniformly scoured two or three times, with an interval between each operation of from six to twenty-four hours. This process leaves the plaster with a close-grained and fairly smooth surface, offering little or no key to the coat that is to follow.
To obtain proper cohesion, however, a roughened face is necessary, and this is obtained by keying the surface with a wire brush or nail float, that is, a hand float with the point of a nail sticking through and projecting about 1/8 inch; sometimes a point is put at each corner of the float.
After the floating is finished to the walls and ceiling, the next part of internal plastering is the running of the cornice, followed by the finishing of the ceiling and walls.
The third and final coat is the setting coat, which should be about 1/8 inch thick. In Scotland it is termed the "finishing coat", and in America the "hard finish coat" or "putty coat". Setting stuff should not be applied until the floating is quite firm and nearly dry, but it must not be too dry or the moisture will be drawn from the setting stuff.
The composition of an interior three coat plaster:
The coarse stuff applied as the first coat is composed of sand and lime, usually in proportions approximating to two to one, with hair mixed into it in quantities of about a pound to two or three cubic feet of mortar. It should be mixed with clean water to such a consistency that a quantity picked up on the point of a trowel holds well together and does not drop.
Floating stuff is of finer texture than that used for pricking up, and is used in a softer state, enabling it to be worked well into the keying of the first coat. A smaller proportion of hair is also used. Fine stuff mixed with sand is used for the setting coat.
Fine stuff, or lime putty, is pure lime that has been slaked and then mixed with water to a semi-fluid consistency, and allowed to stand until it has developed into a soft paste. For use in setting it is mixed with fine washed sand in the ratio of one to three. For cornices and for setting when the second coat is not allowed time to dry properly, a special compound must be used. This is often gauged stuff, composed of three or four parts of lime putty and one part of plaster of Paris, mixed up in small quantities immediately before use. The plaster in the material causes it to set rapidly, but if it is present in too large a proportion the work will crack in setting.