Preparation of Surface for Plastering

Durability of plastering depends on its adhesion with the background. A good-background is one which has enough strength and plain enough for suitable application of plaster. Types of background are (i) Solid Background, (ii) Lathing and (iii) Boards and Slabs.

1. Solid Background

The main characteristic of the solid background is that the nature of surface presented. Whatever may be the condition of solid background, preparation of surface is of prime importance. Following steps are undertaken:

1. The background surface has to be first prepared by cleaning the surface and roughening the mortar joints.
2. Any projection more than 12 mm or the background surface should be removed and cavities and holes are fitted properly so as to obtain an even surface.
3. In order to get a good key for the plaster with the wall surface, all the joints in the masonry are raked out for a depth of at least 12 mm.
4. Dust and loose mortar are brushed out of the raked joints.
5. The surface is rendered free from oily, greasy spots and efflorescence.
6. In case of re-plastering of old works, the smoothness of the surface is destroyed by hacking it with some tool.
7. Whenever concrete surfaces are to be plastered, rough sawn boards should be used. Mechanical keys may be applied to the inside shuttering or metal mesh may be left so as to help in the bonding of the plaster later on.

2. Lath Background

Laths are provided to form a foundation to the plaster work. When lath is used, the number of coats of plaster is to be three. Laths may be of two types, viz., metal laths and wooden laths.

(i) Metal Laths

In the market variety of laths are available. The most commonly used one is the plain expanded metal laths which is prepared from sheets of mild steel. The hyrib which is also a steel lath formed of sheet and perforated with projecting ribs.

Metal laths are supported and fixed with 9–12 mm diameter steel rods or steel channels. The plastering is carried out on both sides. They are fire-resistant.

(ii) Wooden Laths

Strips of well-seasoned wood are used for this purpose. The strips are generally 25 mm wide and 1–1.20 m long. Based on the spacing of timber joints the length of strips is adjusted. The thickness varies from 3 to 12 mm. The thicker strips are used for ceilings whereas thinner strips are used for walls and partitions.

3. Board Background

Boards and slabs have the advantage that a uniform thinner coat of plaster can be given. As the boards have relatively plain surface, they give good results with two coats or even with a single coat plastering. Special treatment may have to be needed for soft type of boards, so as to make better adherence of the plaster.