Cement plastering is commonly used as ideal coating for external and internal surface of wall. Cement plaster is usually applied in a single coat or double coat. Double coat plaster is applied where thickness of plaster is required to be more than 15 mm or when it is required to get a very fine finish. The process of applying a double coat cement plaster on wall surface consists of the following 5 steps.
Step-1-Preparation of surface for plastering
Step-2-Ground work for plaster
Step-3-Applying first coat (or under coat or rendering coat)
Step-4-Applying second coat (or finishing coat or fine coat)
Step by Step Guide for Cement Plastering
Step-1 (Preparation of Surface for Plastering)
Keep all the mortar joints of wall rough, so as to give a good bonding to hold plaster.
Clean all the joints and surfaces of the wall with a wire brush, there should be no oil or grease etc. left on wall surface.
If the surface is smooth or the wall to be plastered is old one, then rake out the mortar joint to a depth of at least 12 mm to give a better bonding to the plaster.
If the projection on the wall surface is more than 12 mm, then knock it off, so as to obtain a uniform surface of wall. This will reduce the consumption of plaster.
If there exist any cavities or holes on the surface, then fill it in advance with appropriate material.
Roughen the entire wall to be plastered.
Wash the mortar joints and entire wall to be plastered, and keep it wet for at least 6 hours before applying cement plaster.
Step-2 (Ground Work for Plaster)
In order to get uniform thickness of plastering throughout the wall surface, first fix dots on the wall. A dot means patch of plaster of size 15 mm * 15 mm and having thickness of about 10 mm.
Dots are fixed on the wall first horizontally and then vertically at a distance of about 2 meters covering the entire wall surface.
Check the verticality of dots, one over the other, 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 plastering being applied.
Step-3 (Applying First Coat or Under Coat or Rendering Coat)
In case of brick masonry the thickness of first coat plaster is in general12 mm and in case of concrete masonry this thickness varies from 9 to 15 mm.
The ratio of cement and sand for first coat plaster varies from 1:3 to 1:6.
Apply the first coat of plaster between the spaces formed by the screeds on the wall surface. This is done by means of trowel.
Level the surface by means of flat wooden floats and wooden straight edges.
After leveling, left the first coat to set but not to dry and then roughen it with a scratching tool to form a key to the second coat of plaster.
Step-4 (Applying Second Coat or Finishing Coat or Fine Coat)
The thickness of second coat or finishing coat may vary between 2 to 3 mm.
The ratio of cement and sand for second coat plaster varies from 1:4 to 1:6.
Before applying the second coat, damp the first coat evenly.
Apply the finishing coat with wooden floats to a true even surface and using a steel trowel, give it a finishing touch.
As far as possible, the finishing coat should be applied starting from top towards bottom and completed in one operation to eliminate joining marks.
After completion of the plastering work, it is kept wet by sprinkling water for at least 7 days in order to develop strength and hardness.