This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
When the lathing is finished, the work is either laid or pricked up, according as it is to be finished with 1, 2, or 3 coats. " Laying " is a thick coat of coarse stuff, or lime and hair, brought to an even surface with the trowel only; for this the mortar must be well tempered, and of moderate consistence, thin or moist enough to pass readily through between the laths, and bend with its own weight over them, and at the same time stiff enough to leave no danger that it will fall apart, a contingency, however, that in practice frequently occurs, in consequence of badlly-com-posed or badly-tempered mortar, unduly close lathing, or sufficient force not having been used with properly consistent mortar to force it through and form keys. If the work is to be of 2 coats, i. e. "laid and set," when the laying is sufficiently dry, it is thoroughly swept with a birch broom or scratcher to roughen its surface, and then the " set," a thin coat of fine stuff is put on. This is done with the common trowel alone, or assisted by a wetted hogs'-bristle brush, which the workman uses with his left hand to strike over the surface of the set, while he presses and smooths it with the trowel in his right.
If the laid work should have become very dry, it must be slightly moistened before the set is put on, or the latter, in shrinking, will crack and fall away. This is generally done by sprinkling or throwing the water over the surface from the brush.
For " floated," or 3-coat work, the first, or "pricking up," is roughly laid on the laths, the object being to make the keying complete, and form a layer of mortar on the laths to which the nest coat may attach itself. It must, of course, be kept of equal thickness throughout, and should stand about |-| in. on the surface of the laths. When it is finished, and while the mortar is still quite moist, the plasterer scratches or scores it all over with the end of a lath or the scratcher. These scorings should be made as deep as possible, without laying bare the laths; and the rougher their edges are the better, as the object is to produce a surface to which the next coat will readily attach itself.
When the "pricked-up" coat is so dry as not to yield to pressure in the slightest degree, preparations may be made for the " floating." Ledges, or margins of lime and hair about 6 in. in width, and extending across the whole breadth of a ceiling, or height of a wall or partition, must be made in the angles or at the borders, and at distances of about 4 ft. apart throughout the whole extent. These must be straight with one another and be proved in every way by the application of straight-edges. Technically these ledges are termed " screeds." The screeds are gauges for the rest of the work; for when they are ready, and the mortar in them is a little set, the interspaces are filled up flush with them, and a darby float, or long straight-edge, being made to traverse the screeds, all the stuff that projects beyond the line is struck off, and thus the whole is brought to a straight and perfectly even surface.
To perfect the work the screeds on ceilings should be levelled, and on walls and partitions plumbed. When the floating is sufficiently set and nearly dry, it is brushed with a birch broom as before described, and the third coat, or "set," is put on. This, for a fine ceiling that is to be whitened or coloured, must be of what plasterers call " putty "; but if it is to be papered, ordinary fine stuff, with a little hair in it, will be better. Walls and partitions that are to be papered are also faced with fine stuff, or rough stucco, but for paint the set must be of bastard stucco trowelled.
Plastering in external walls requires the addition of some hydraulic cement to the mortar. The scaling of plaster liable to the action of water and frost is said by Cameron to be prevented by mixing sawdust with the mortar.