Lime Plastering is the process of covering the surfaces by lime plaster or lime mortar in various proportions depending upon the nature of work and the number of coats to be applied. These proportions of lime and sand generally adopted for either external work or internal work. Sometimes, an adequate quantity of cement is added with the lime plaster to increase its strength particularly for external work. The process of lime plastering is explained below in steps.

Step1. Preparation of surface for plastering:

When a surface is to be plastered, the surface is prepared in the following manner.

(i) All the mortar joints of wall to be plastered are left rough and projecting, so as to give a key or hold to the plaster. All the joints and surfaces are well cleaned with a wire brush and ensured that they are free from oil, grease, etc. if the surface is smooth or the wall to be plastered is old one, then the mortar joints are raked out at least to a depth of 12 mm to give a key to the plaster.

(ii) The projections more than 12 mm over the surface are knocked off so as to obtain a uniform surface of wall and also to reduce the consumption of plaster.

(iii) Similarly, all the cavities and holes inside the surface are properly filled up in advance.

(iv) All wall work to be plastered is roughened.

(v) Finally, the mortar joints and surfaces of wall are well washed, wetted with water and kept for at least 6 hours before plastering.

Step2. Ground work for plaster:

For the application of plaster, a ground work is required, which is described below:

In order to obtain a true surface and uniform thickness, vertical strips called screeds or bands are formed on the wall surface by fixing dots (i.e., patches of plaster). These dots are first of all applied horizontally and then vertically at a distance of about 2 m, covering the entire wall surface. These dots are fixed by laying a small quantity of plaster on the surface in the forms of squares (15 cm x 15 cm) having thickness of about 10 mm.

The vertically of dots, one over the other, is checked by means of plumb-bob. After fixing dots, the vertical strips of plaster, known as screeds, are formed in between the dots. These screeds serve as the gauges for maintaining even thickness of plaster being applied. Mortar is then applied on the wall between ground work of screeds so prepared.

Step3. Application of plaster coats:

Plaster may be applied in one, two or three coats. The procedure of carrying out the plastering in three coats.

(i) First Coat (or Rough Course of Plastering or Rendering coat). It is usual to provide an average thickness of first coat of plaster equal to 12 mm on brick or ashlar masonry and 20 m on rubble masonry. The larger thickness of rubble masonry being due to the roughness of its surface and the necessity of providing a minimum covering of 6 mm of mortar to rubble.

The first coat (i.e., rendering coat) of plaster is generally applied by dashing against the wall surface between the screeds mentioned above. It is then sprinkled frequently with sufficient quantity of water and rubbed well by means of floats. (This surface can be finally finished and left for curing for at least 7 days, if plastering is required to be carried only in a single coat).

If a second coat, called floating coat is to be applied, the surface of the first coat is left exposed to air for a period of 2 days to set but not to dry. After this period the surface of first coat is swept clear of any dust or loose particles, sprinkled with water and well beaten with thin strips of bamboo or cane. The surface of the first coat is kept wet till the second coat is applied.

(ii) Second Coat or Floating Coat. The second coat is applied after preparing the surface of the first coat as mentioned above. The second coat is spread out uniformly with trowels. It is pressed and rubbed with wooden straight edge, to obtain the desired surface. It is finally finished by slightly sprinkling water over the plastered surface rubbing it with the floats. The thickness of second coat is usually kept between 6 to 9 mm.

(iii) Third Coat or Final Coat or Finishing Coat. The coat is applied after 5 days of the second coat. This coat, is consisting of cream of white or fat lime (called plasterer’s putty) and fine white sand in the ratio of 1 : 2, is laid in thickness of 3 mm with straight plane and rubbed with straight edge. The surface is well rubbed with wooden float and then finally finished with trowel to obtain the desired surface. A polishing stone is very often used to obtain a fine polished surface.


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Plastering

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