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Preparation of background: For plastering new surfaces, all masonry joints should be racked to a depth of 10 mm in brick masonry and 15 mm in stone masonry for providing key to the plaster. All mortar droppings and dust and laitance (in case of freshly laid concrete) should be removed with the help of stiff wire brush. Any unevenness is leveled before rendering is applied. For finishes applied in three coats, local projections should not be more than 10 mm proud of general surface and local depressions should not exceed 20 mm. For two coat plasters these limitations are 5 mm and 10 mm respectively. The surface should be washed with clean water and kept damp uniformly to produce optimum suction. In no case the surface should be kept soaked with water so as to cause sliding of mortar before it sets or kept less wet to cause strong suction which withdraws moisture from mortar and makes it weak, porous and friable. If plaster is to be applied on old surface, all dirt, scool, oil, paint etc. should be cleaned off. Loose and crumbling plaster layer should be removed to its full thickness and the surface of the background should be exposed and joints properly racked. The surface should be washed and kept damp to obtain optimum suction.
- Lime Plaster
Lime plaster is applied either in three coats or in two coats. Before the application of plaster, the background should be prepared .
- Three – coat plaster
In the 3- coat plaster, the first coat is known as rendering coat second coat known as floating coat and the third coat is known as setting coat or finishing coat.
- Application of rendering coat
The mortar is forcibly applied with mason’s trowel and pressed well into join and over the surface. The thickness of coat should be such as to cover all inequalities of the surface; normal thickness is 12 mm. This is allowed to slightly harden, and then scratched criss – cross with the edge of trowel or with devil float. The spacing of scratches may be 10 cm. The surface is left to set at least for 7 days. During this period, the surface is cured by keeping it damp and then allowed to dry completely.
- Application of floating coat
The rendering coat is cleaned off all dirt, dust and other loose mortar dropping: It is lightly wetted. Patches 15 cm x 15 cm or strips 10 cm wide are applied at suitably spacing to act as gauges. The mortar is then thrown with mason’s trowel, spread and rubbed to the required plain surface with wooden float. The surface so obtained should be true in all directions. In case of lime – sand plaster, the finishing coat is applied immediately. In the case of lime – surkhi plaster, the floating coat is allowed to slightly set and then lightly beaten criss – cross with floats edge at close spacing, of 4 cm. it is then cured to set completely for 10 days and then allowed to dry out completely. In either case, the thickness of coat varies from 6 to 9 mm.
- Application of finishing
In the case of lime – sand mortar the finishing coat is applied immediately after the floating coat. The finishing coat consists of cream of lime called neeru or plaster’s putty, having lime cream and sand in the ratio of 4:1 applied with steel trowel and rubbed and finished smooth. The rubbing is continued till it is quite dry. It is left for 1 day, and then curing is done for at least 7 days. In the case of lime – surkhi mortar, the finishing coat is applied 7 days after the floating coat, after cleaning the surface of all dirt, dust and mortar dropping and after fully wetting the surface of previous coat. The finishing coat is rubbed hard and finished smooth.
- Two – coat plaster
In the case of lime – sand mortar the finishing coat is a combination of the rendering floating coats of ‘three – coat plaster’ and is done under one continuous operation except that the scratching of rendering coat, as specified in the three – coat plaster, is not done. The total thickness may be about 12 mm. the finishing is then applied in a manner similar to the three – coat plaster.
- Cement Plaster And Cement-Lime Plaster
Cement plaster is applied either in two coats or in three coats: For inferior work, single coat plaster is sometimes provided.
- Two –coat plaster: The following procedure is adopted in two coat plaster.
- The background is prepared by racking the joint to a depth of 20 mm, cleaning the surface and well – watering it.
- If the surface to be plastered is very uneven, a preliminary coat is applied to fill up the hollows, before the first coat.
- The first coat or rendering coat of plaster is applied, the thickness being equal to the specified thickness of plaster less 2 to 3 mm. In order to maintain uniform thickness of plaster, screeds are formed of plaster on wall surface by fixing dots of 15 cm x 15 cm size. Two dots are so formed in vertical line, at s distance of about 2m, and are plumbed by means of a plumb bob. A vertical strip of mortar, known as screed, is then formed. A number of such vertical screeds are formed at suitable spacing. Cement mortar is then applied in the surface between the successive screeds and the surface is properly finished.
- Before rendering hardens, it is suitable worked to provide mechanical key for the final or finishing coat. The rendering coat is travelled hard forcing mortar into joints and over the surface. The rendering coat is kept wet for at least 2 days, and then allowed to dry completely.
- The thickness of final or finishing coat may vary between 2 and 3 mm. before applying the final coat, the rendering coat is damped evenly. The final coat is applied with wooden floats to a true even surface and finished with steel trowels. As far as possible, the finishing coat should be applied starting from top towards bottom and completed in one operation to eliminate joining marks.
- Three – coat plaster: The procedure for applying three – coat plaster is similar to the two – coat plaster except that an intermediate coat, known as floating coat is applied. The purpose of this coat of plaster is to bring the plaster to an even surface. The thickness of rendering coat, floating coat and finishing coat are kept 9 to 10 mm, 6 to 9 mm and 2 to 3 mm respectively. The rendering coat is made rough. The floating coat is applied about 4 to 7 days after applying the first coat. The finishing coat may be applied about 6 hours after the application of floating coat.
- Single – coat plaster: This is used only in inferior quality work. It is applied similarly as two coat plaster except that the rendering coat, as applied for two – coat plaster, is finished off immediately after it has sufficiently hardened.
Plaster On Lath
Laths are adopted to provide foundation for plaster work. Laths are also provided for plastering thin partition walls and for plastering ceilings. Laths may be of two types: (i) Wooden laths and (ii) Metal laths.
Wooden laths used for plastering over wooden partition walls and ceilings, are in the form of well – seasoned wooden strips 25 mm wide and 1 to 1.2 m long. These strips are fixed in parallel lines with clear spacing of 10 mm, and secured to the surface with galvanized iron nails.
Metal laths are available under various patent names. The plain expanded metal lath (Exmat) is commonly used. Metal laths are fixed to the surface by G.I. staples. In case of concrete or masonry surface, wooden plugs have to be embedded for fixing the lath. After fixing the lath, the surface is plastered, usually in three coats. Cement mortar is used.
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