Methods

External plastering
Main article: Stucco
Stucco is a term loosely applied to nearly all kinds of external plastering, whether composed of lime or of cement. At the present time it has fallen into disfavor, but in the early part of the 19th century a great deal of this work was done. Cement has largely superseded lime for this work. The principal varieties of stucco are common, rough, trowelled and bastard. .
Common stucco for external work is usually composed of one part hydraulic lime and three parts sand. The wall should be sufficiently rough to form a key and well wetted to prevent the moisture being absorbed from the plaster.
Rough stucco is used to imitate stonework. It is worked with a hand float covered with rough felt (a stiff bristled brush can also be used), which forms a sand surface on the plaster. Lines are ruled before the stuff is set to represent the joints of stonework.
Trowelled stucco, the finishing coat of this work, consists of three parts sand to two parts fine stuff. A very fine smooth surface is produced by means of the hand float.
Bastard stucco is of similar composition, but less labor is expended on it. It is laid on in two coats with a skimming float, scoured off at once, and then trowelled.
Colored stucco: lime stucco may be executed in colors, the desired tints being obtained by mixing with the lime various oxides. Black and grays are obtained by using forge ashes in varying proportions, greens by green enamel, reds by using litharge or red lead, and blues by mixing oxide or carbonate of copper with the other materials.
Main article: Roughcast
Roughcast or pebbledash plastering is a rough form of external plastering in much use for country houses. In Scotland it is termed "harling". It is one of the oldest forms of external plastering. In Tudor times it was employed to fill in between the woodwork of half-timbered framing. When well executed with good material this kind of plastering is very durable.
Roughcasting is performed by first rendering the wall or laths with a coat of well-haired coarse stuff composed either of good hydraulic lime or of Portland cement. This layer is well scratched to give a key for the next coat. The second coat is also composed of coarse stuff knocked up to a smooth and uniform consistency. Two finish two techniques can be used:
dry dash: while the first coat is still soft, gravel, shingle or other small stones are evenly thrown on with a small scoop and then brushed over with thin lime mortar to give a uniform surface. The shingle is often dipped in hot lime paste, well stirred up, and used as required.
wet dash: the traditional roughcast, harling the scratch or undercoat is left to cure and in the final coat the gravel/agrigate is mixed with the lime and sand and thrown on with the plaster spoon/scoop.

Sgraffito (scratched ornament)
Main article: Sgraffito
Sgraffito is the name for scratched ornament in plaster. Scratched ornament is the oldest form of surface decoration, and is much used on the continent of Europe, especially in Germany and Italy, in both external and internal situations.
Properly treated, the work is durable, effective and inexpensive. A first coat or rendering of Portland cement and sand, in the proportion of one to three, is laid on about an inch thick; then follows the color coat, sometimes put on in patches of different tints as required for the finished design. When this coat is nearly dry, it is finished with a smooth-skimming, 1/12 to 1/8 inches thick, of Parian, selenitic or other fine cement or lime, only as much as can be finished in one day being laid on.
Then by pouncing through the pricked cartoon, the design is transferred to the plastered surface. Broad spaces of background are now exposed by removing the finishing coat, thus revealing the colored plaster beneath, and following this the outlines of the rest of the design are scratched with an iron knife through the outer skimming to the underlying tinted surface.
Sometimes the coats are in three different colors, such as brown for the first, red for the second, and white or grey for the final coat. The pigments used for this work include Indian red, Turkey red, Antwerp blue, German blue, umber, ochre, purple brown, bone black or oxide of manganese for black. Combinations of these colors are made to produce any desired tone.

Coats
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 tree 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.
The hard cements used for plastering, such as Parian, Keene's, and Martin's, are laid generally in two coats, the first of cement and sand 1/2 to 3/4 inches in thickness, the second or setting coat of neat cement about 1/8 inch thick. These and similar cements have gypsum as a base, to which a certain proportion of another substance, such as alum, borax or carbonate of soda, is added, and the whole baked or calcined at a low temperature. The plaster they contain causes them to set quickly with a very hard smooth surface, which may be painted or papered within a few hours of its being finished.
Main article: Cement render
In Western Australia plaster or cement render that is applied to external brickwork on dwellings or commercial buildings can be one or two coats.
Materials used in the render are commonly sand of a light yellow colour with little clay content with fine to coarse grains or sand. Sand finish is the common term used for external render and may be one or two coats, the better being two coat as it gives a more consistent finish and less chance of becoming drummy or cracking.
In two coat render a base coat is applied with a common mix of 4 parts sand to one part cement and one part dehydrated lime and water to make a consistent mortar. Render is applied using a hawk and trowel and pushed on about 12 mm thick to begin. Most plasterers use a tbar to screed the walls until it is plumb straight and square. The wall is scratched after screeding is complete to give key to second coat.
The second can be slightly weaker or the same 5/1/1 and maybe a water proofer in the mix added in the water to minimize effloresence (rising of salts). Some plasterers used lime putty in second coat instead of dehydrated lime in the render. The mortar is applied to about 5 mm thick and when the render hardens is screeded off straight. A wood float or plastic float is used to rub down the walls.
Water is splashed on walls and immediately rubbing the float in a circular or figure 8 motion (figure of 8 can leave marks).After the work area is all floated, the finishing with a sponge using the same method as floating with wood float. Bringing sand to the surface.
Most plasterers use a hose with a special nozzle with a fine mist spray to dampe walls when rubbing up (using a wood float to bring a consistent finish). This method using a hose brings a superior finish and more consistent in colour as there is more chance in catching the render before it has a chance to harden too much.

Moldings
Plain, or unenriched, moldings are formed with a running mold of zinc cut to the required profile.
For a cornice molding two running rules are usual, one on the wall, the other on the ceiling, upon which the mold is worked to and from by one workman, while another man roughly lays on the plaster to the shape of the molding. The miters at the angles are finished off with joint rules made of sheet steel of various lengths, three or four inches (102 mm) wide, and about one-eighth inch thick, with one end cut to an angle of about 30°. In some cases the steel plate is let into a stock or handle of hardwood.
Enrichments may be moldings added after the main outline molding is set, and are casted in molds made of gelatin or plaster of Paris.

Cracks
Cracks in plastering may be caused by settlement of the building, by the use of inferior materials or by bad workmanship.
However, due to none of these, cracks may yet ensue by the too fast drying of the work, caused through the laying of plaster on dry walls which suck from the composition the moisture required to enable it to set, by the application of external heat or the heat of the sun, by the laying of a coat upon one which has not properly set, the cracking in this case being caused by unequal contraction, or by the use of too small a proportion of sand.
Traditionally, crack propagation was arrested by stirring chopped horsehair thoroughly into the plaster mix.

Slabs
For partitions and ceilings, plaster slabs are used to finish quickly. For ceilings metal lathing require simply to be nailed to the joists, the joints being made with plaster, and the whole finished with a thin setting coat or slab. In some cases, with fireproof ceilings, for instance, the methal lathing are hung up with wire hangers so as to allow a space of several inches between the soffit of the concrete floor and the ceiling. For partitions methal laths are grouted in with semi-fluid plaster. Where very great strength is required the work may be reinforced by small iron rods through the slabs. This forms a very strong and rigid partition which is at the same time fire-resisting and of lightweight, and when finished measures only from two to four inches (102 mm) thick.So strong is the result that partitions of this class only two or three inches (76 mm) thick were used for temporary cells for prisoners at Newgate Gaol during the rebuilding of the new sessions house in the Old Bailey in London.
The slabs may be obtained either with a keyed surface, which requires finishing with a setting coat when the partition or ceiling is in position, or a smooth finished face, which may be papered or painted immediately the joints have been carefully made.

Fibrous plaster
Fibrous plaster is given by plasterers the suggestive name "stick and rag", and this is a rough description of the material, for it is fibrous composed of plaster laid upon a backing of canvas stretched on wood. It is much used for moldings, circular and enriched casings to columns and girders and ornamental work, which is worked in the shop and fixed in position.
Desachy, a French modeler, took out in 1856 a patent for "producing architectural moldings, ornaments and other works of art, with surfaces of plaster," with the aid of plaster, glue, wood, wire, and canvas or other woven fabric.
The modern use of this material may be said to have started then, but the use of fibrous plaster was known and practiced by the Egyptians long before the Christian era; for ancient coffins and mummies still preserved prove that linen stiffened with plaster was used for decorating coffins and making masks. Cennino Cennini, writing in 1437, says that fine linen soaked in glue and plaster and laid on wood was used for forming grounds for painting.
Canvas and mortar were in general use in Great Britain up to the middle of the last century. This work is also much used for temporary work, such as exhibition buildings