The process of applying a thin coat on the concrete structure to protect from the weather, temperature to increase the life span of the structure is known as plastering.
Methods of plastering
- The plaster may be applied in one or more coats, but the thickness of a single coat should not exceed 12 mm.
- In the case of the inferior or cheaper type of construction, the plaster may usually be one coat. For the ordinary type of construction, the plaster is usually applied in two coats, whereas for a superior type of works it is applied in three coats.
- The final setting coat should not be applied until the previous undercoat is almost dry.
- Before applying the next coat of plaster the previous plastered surface should be scratched or roughened to form a key with an overcoming layer.
- In plastering plaster mix is either applied by throwing it with great force against the wall or by pressing it on the surface.
Methods of Cement Plastering
Cement plastering is commonly used as an ideal coating for the external and internal surfaces of the wall. Cement plaster is usually applied in a single coat or double coat. Double coat plaster is applied where the 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 the wall surface consists of the following steps.
Step- I Preparation of surface for plastering
Step-2 Ground work for plaster
Step-3 Applying first coat (or undercoat or rendering coat)
Step-4 Applying second coat (or finishing coat or fine coat)
Let us discuss steps for methods of plastering one by one in detail below.
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 the wall surface.
- If the surface is smooth or the wall to be plastered is old, 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 the 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 plastering
- In order to get the uniform thickness of plastering throughout the wall surface, first, fix dots on the wall.
- A dot means a patch of plaster of size 150 mm * 150 mm and having a 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. the thickness of plastering being applied.
Step-3 Applying first coat or under coat or rendering coat
- In the case of brick masonry, the thickness of first coat plaster is in general 12 mm and in the 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 1:4.
- Apply the first coat of plaster between the spaces formed by the screeds on the wall surface. This is done by means of the 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 the 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 to start from the top towards the 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.
I hope this article on “Methods of Plastering” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Ranjeet Sahani