plastering and painting

Building Materials and Construction

6. PLASTERING, WHITE WASHING AND PAINTINGPlasteringApplying mortar coats on the surfaces of walls, columns, ceiling etc. to get smooth finish is termed as plastering. Mortar used for plastering may be lime mortar, cement mortar or lime-cement mortar. Lime mortar used shall have fat lime to sand ratio of 1: 3 or 1: 4. If hydraulic lime is used mix proportion (lime: sand) is 1: 2. Cement mortar of 1: 4 or 1: 6 mix is very commonly used for plastering, richer mix being used for outer walls. To combine the cost effectiveness of lime mortar and good quality of cement mortar many use lime-cement mortar of proportion (cement: lime: sand) of 1: 1: 6 or 1: 1: 8 or 1: 2: 8.

Plastering is done to achieve the following objects1. To protect the external surfaces against penetration of rain water and other atmospheric

agencies2. To give smooth surface in which dust and dirt cannot lodge3. To give decorative effect4. To protect surfaces against vermin.5. To conceal inferior materials or defective workmanship

Requirements of good plaster:1. It should adhere to the background and should remain adhered during all variations of the

climatic changes.2. It should be cheap and economical3. It should be hard and durable.4. It should be possible to apply it during all weather conditions5. It should be effectively check the entry or penetration of moisture from the surface6. It should possess good workability

Materials used for plasteringLime plaster: Lime plaster is a mixture of calcium hydroxide and sand (or other inert fillers) in 1:1 ratio. Carbon dioxide in the atmosphere causes the plaster to set by transforming the calcium hydroxide into calcium carbonate (limestone). In order to improve building properties of lime plaster, gugal (a kind of fragrant gum) @ 1.6 kg/m3 of mortar, is added when the mortar is being ground. In order to improve adhesive and tensile properties of lime mortar, sometimes, small quantities of chopped hemp (i.e., vegetable fibres) @ 1kg/m3 are added to the lime mortar. The lime mortar thus prepared is usually kept for 2 days before use.

Cement plaster: Cement and sand in required proportions (1:3 or 1:4) are first thoroughly mixed in dry conditions and then water is added to form a paste of required consistency. Generally, it is

Compiled by Dr. Prashanth J. and Dr. Harish N.


mixture of sand, Portland cement and water mixed in a suitable proportions. This prepared mortar for plastering should be consumed within 30 minutes after the addition of water.

Gypsum plaster (plaster of Paris): Gypsum plaster, or plaster of Paris, is produced by heating gypsum to about 1500C. When the dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes but not fully set for 72 hours.

Water proof plaster of mortar: This mortar consists of 1 part of cement, 2 parts of sand and pulverised alum @ 12kg/m3 of sand. To this dry mix, the soap water containing about 75g of soft soap/lt of water, is added to obtain the waterproof mortar.

Heat resistant plaster: Its purpose is to replace conventional gypsum plasters in cases where the temperature can get too high for gypsum plaster to stay on the wall. Heat resistant plaster should be used in cases where the wall is likely to exceed temperatures of 500C. Heat resistant plaster is a building material used for coating walls and chimney breasts.

Procedure for Cement Plastering on Wall SurfaceCement 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 4 steps.

Step-1-Preparation of surface for plasteringStep-2-Ground work for plasterStep-3-Applying first coat (or under coat or rendering coat)Step-4-Applying second coat (or finishing coat or fine coat)

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.

Dots and Screeds

STEP-3: Applying First Coat or Under Coat or Rendering Coat In case of brick masonry the thickness of first coat plaster is in general 12 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.

Defects in PlasterThe following defects may arise in plaster work.

1. Blistering of plastered surface: This is the formation of small patches of plaster swelling out beyond the plastered surface, arising out of late slaking of lime particles in the plaster.

2. Cracking: Cracking consists of formation of cracks or fissures in the plaster work resulting from the following reasons. a. Imperfect preparation of backgroundb. Structural defects in buildingc. Discontinuity of surfaced. Movements in the background due to its thermal expansion or rapid dryinge. Movements in the plaster surface itself, either due to expansion or shrinkage.f. Excessive shrinkage due to application of thick coatg. Faulty workmanship.

3. Efflorescene: It is the whitish crystalline substance which appears on the surface due to presence of salts in plaster making materials as well as building materials like bricks, sand, cement etc and even water. This gives a very bad appearance. It affects the adhesion of paint with wall surface. Efflorescence can be removed to some extent by dry bushing and washing the surface repeatedly.

4. Flaking: It is the formation of very loose mass of plastered surface, due to poor bond between successive coats.

5. Peeling: It is the complete dislocation of some portion of plastered surface, resulting in the formation of a patch. This also results from imperfect bond.

6. Popping: It is the formation of conical hole in the plastered surface due to presence of some particles which expand on setting

7. Rust Stains: These are sometimes formed when plaster is applied on metal laths 8. Uneven surface: This is obtained purely due to poor workman ship.

PointingThe art of finishing the mortar joints in exposed brick or stone masonry with suitable cement or lime mortar, in order to protect the joints from weather effects and also to improve the



appearance of building structure. Often an entire wall or even a whole structure is pointed because defective points cannot easily be detected, and adjacent joints may also be in need of repair. The mortar is packed tightly in thin layers and tooled to a smooth, concave, finished surface.

Scope of pointing Maintaining the joints of the structures. Pointing being cheap can be adopted in places of low rainfall. Where the natural beauty if materials viz., stone blocks, bricks etc., is desired to be

exhibited. Gives resisting power to the bricks and stones used in construction towards weather

conditions.

Method of pointing All the mortar joints (on the masonry face required to be pointed) are raked out by a

special pointing tool to a depth of 15 to 20 mm, so as to provide an adequate key for the fresh mortar used for pointing.

All the loose mortar and dust are removed by brushes. The joints and wall surface are washed with clean water, and then kept wet for few hours. The joints so prepared, are filled with suitable mortar with a small trowel. The mortar is

well pressed into the joints to form a close contact with the old interior mortar joints. All excess mortar sticking to the sides is scraped away.

The finished pointing work is kept wet for about 3 days when lime mortar is used for pointing and for 10 days when cement mortar is used for pointing.

White washingWhitewash for walls can be lime or paint-based, and can be applied to brick, concrete and other wall surfaces. Pigment can be changed by adding colors to the mixture. Each change in the mixture should be tested, however, before applied to a whole wall. Pigments change when they become completely dry; if you do not test the area first you may have unexpected and undesirable results.

Cleaning the walls first will help to ensure an even finish. More than one type of cleaner may be required if you have to remove grease as well as dirt. It is very important to clean well or it will have splotchy areas where there is dirt. If cleaning concrete walls, use a wire cleaning brush for really tough areas but do not chip away the concrete. Thoroughly apply whitewash with a paint brush. The whitewash will apply better to a moist surface, so wet the walls first and allow it to dry partially before you start. More than one application of whitewash is usually needed. Apply thin even strokes until the wall is covered evenly. By the time that the third coat is begun the



brush strokes should no longer be visible. One tip to remember is that each coat needs to dry thoroughly before starting the next one. Drying time will vary based on how humid the area is.

Whitewashed wall surface

PaintingPaints are coatings of fluid materials which are applied as a final finish to all surfaces such as walls, ceilings, wood work, metal work etc. The process of application of paint as a coating material is called painting. The ingredients of paints and their functions are discussed below.

Ingredients of paintBase: It is the main ingredient of paint. It hides the surface to be painted and imparts durability to the painted surface. Commonly used bases are white and red lead, zinc and iron oxides.Carrier: The carrier is a liquid which acts as a binder and helps to spread the ingredients uniformly on the painted surface. Linseed, Tung, Poppy, Sunflower oil etc. are some of the commonly used carriers.Drier: Driers are metallic compounds used in small quantities for accelerating the process of drying of paint film. Litharage, Lead acetate, Manganese dioxide and Cobalt are the driers commonly used.Solvent: Solvent or thinner is a liquid which thins the paint film so that it can be easily applied on the surface and it helps the paint in penetrating through porous surfaces. Turpentine, petroleum, spirit and highly solvent naptha are commonly used as solvent.Colouring pigments: To obtain desired colour and shades of the paint, colouring pigments are added.Characteristics of paint

i. It should dry quickly.ii. It should be easy to apply and cover large surface area.iii. It should not be harmful to health.



iv. It should not react with the wall or wooden material.v. The paint should not be affected by weathering actions of the atmosphere.vi. The paint should form a hard and durable surface.vii. The paint should be fairly cheap and economical.

Uses of painti. Protects the surfaces from the weathering effects of the atmosphere and actions by other

liquid, fumes and gases.ii. Prevent the decay of wooden members.iii. Prevent the corrosion of metallic surface.iv. Provide a smooth surface for easy cleaning.v. Provide pleasing, colourful and decorative appearances to the surfaces.vi. Increase the life of the buildings.vii. It covers the defective workmanship and materials.

Types of paints1. Oil paint: This is an ordinary paint. It is generally applied in three coats namely priming

coat, undercoats and finishing coats. It is very cheap and possesses opacity and low gloss.2. Enamel paint: This paint consists of white lead or zinc white ground in a small quantity

of oil and mixed with petroleum spirit and resinous matter. It is available in different colours.

3. Emulsion paint: It consists of polyvinyl acetate and synthetic resin as binding materials. It is easy to apply and dries quickly. Colour of paint is retained for a long period. It can be cleaned by washing with water.

4. Aluminum paint: This consists of aluminum powder suspended in spirit varnish or oil varnish. After the application of the paint the spirit becomes evaporated and the aluminum is deposited on the surface as thin layer. Aluminum paint has the advantage such as visibility in darkness, imperviousness to moisture, good appearance to the surface, high electrical resistance and resistance to heat and sunlight.

5. Bituminous paint: It is prepared by dissolving asphalt or vegetable pigment in any type of oil or petroleum. It presents a black appearance. This type of paint is used to paint iron grills.

6. Cement paint: This type of paint is prepared by using white cement, pigment accelerators and other additives. It is available in various colour and available in dry form. It is water-dilutable paint. It is mainly applied on brick and cement surface and resists the penetration of moisture. It gives long life.

7. Plastic paint: The paint contains necessary variety of plastics. It is available in the market under different trades and colours. The plastic paint can be applied either by brush or by spray painting. It gives recreation, pleasing and good appearance.



Method of painting old wood workPreparation of surface for painting old wood work:

i. The surface of the old wood work should be cleaned by using sand paper.ii. Old surface should be cleaned by using a litre of water mixed with 200 gm of caustic

soda and lime to remove the old colours.iii. Nails if any should be removed.

Procedure of painting old wood surface:i. Priming coat is applied on the surface to reduce quantity of oil paint.ii. After drying of the priming coat the first coat of paint is applied on the surface.iii. After drying of the first coat of paint second and third coats will be applied on the surface

subsequently.

Method of painting new wood work1. Preparation of surface for painting: The surface to be painted should be carefully cleaned

and rendered smooth.2. Knotting: Knotting is of three types. The commonly adopted two types are,

i. Ordinary knotting: This is applied in two coats. The first coat consists of hot lead ground in water with strong glue added to it. After drying, the second coat is, applied which consists of red lead ground in oil and thinned with turpentine. By doing this knots are removed.

ii. Patent knotting: This consists of two coats of varnish which is made by dissolving shellac in methylated spirit. Knots are removed by applying this mixture. Boiled lime is also used to remove knots.

3. Priming coat or first coat: After knotting the priming or first coat is applied to the whole surface of the wooden article to be painted.

4. Stopping: After priming the process of stopping is carried out. After stopping dries, the surface is well rubbed down with emery sheets and cracks if any filled with putty. This is known as stopping.

5. Application of succeeding coat: Painting is done by good quality brush. Finally, two coats of paint are applied one after the other. The second coat is applied only after the first coat is dried thoroughly. While painting, the brush is held at right angles to the surface and only the hairs should touch the surface. If necessary after drying of second coat the third coat is also applied.



Defects in wood

Method of painting old iron and steel worksPreparation of the surface:

i. The iron and steel work to be painted is made free from all mill scale, loose rust and grease by using iron brushes.

ii. The old paint on the iron work should be cleaned by using phosphoric acid.Procedure of painting:

i. The surface to be painted is cleaned with emery sheet.ii. Holes if any on the surface should be filled with metallic paste. Then as the priming coat,

red oxide is applied on the surface. This is cheap. Painting adheres well on it.

Method of painting new iron and steel worksPreparation of surface: The iron or steel work to be painted is made free of all mill scale, loose rust and grease by using iron brushes. The loosened scales are removed by a jet of hot water, followed by a bath of hydrated lime and petroleum. Cleaned metal surface is finally treated with phosphoric acid.

Application of coats of paint: Surface of steel work is cleaned and dried. After this a priming coat is applied. After this has dried the second coat of paint is applied by using brush or spray. The completed paint surface should be smooth and uniform.

Method of cement painting on wall surfacePreparation of surface:

i. Newly constructed wall surface is wet. The wall surface should not be painted until it is completely dry. Time duration of drying depends on the ventilation and weather condition. Painting on a freshly plastered surface should be avoided for atleast 3 to 6 months.

ii. The wall surface should be prepared well before painting to remove free lime or cement in the wall. Otherwise, it will discolour the paint and destroy its oil. To avoid this priming coat is applied which does not consist of chemicals that reacts with alkaline.



iii. Painting should be done only after the surface is given a cost of sizing (glue mixed with water) to fill up the cracks.

iv. White patches that occur on the plastered surface can be removed by water washing with wire brush. If white patches appear again painting should be delayed.

v. While painting, if any change occurs antiseptic washing treatment is given.

Application of cement paint:i. Initial coat of white cement is applied on the exterior walls.ii. After curing is completed first coat primer is applied.iii. Above the primer coat, two coats of cement paint are applied. Hence, incomplete

portions are finished by the second coat.iv. Finishing coat is applied in the opposite direction of the previous coat.

Important points to be considered while paintingi. Brush: Brush used for painting should be of fibre bristles and not by using horse hairs.

Only then more quantity of paint could be taken by the brush which will facilitate for easier painting. The brush should be cleaned by kerosene after the application of the paint.

ii. Paints: Ready mixed paints available in the market can be used. Since it is highly expensive two or more colours of paint are kept in stock and the required colour may be changed according to the situation. For this, paint is taken in a solid state in a container and it is stirred well by adding linseed oil. Then by adding pigments for a required colour and suitable ingredients the paint is prepared.

iii. Knotting: Before painting the wooden surface the process of removing knot on the surface by hiding it and painting over it is called knotting. Paint should be applied only after knotting.

iv. Priming coat: A coat of cheap rate paint is applied.v. Stopping: After completing the first coat it should be rubbed by emery sheet and pumice

stone. Then stopping is carried out and second coat is applied.vi. Coating: generally painting should be applied in 2 or 3 coats. Application of first coat is

called primary coat. Second coat is called base coat. Third coat is called final coat.vii. Spray painting: Instead of painting with ordinary brush the surface can also be painted

with spray pistol by passing the air with pressure. This is called as “spray painting”. Spray painting is better than painting with brush. It includes special features like speed, quality, economy and labour.

Defects in painting, causes and precautionsSl. No.

Defects Causes Precautions

1 Blistering It is caused due to the Avoiding drops of water, water



trapping of water vapor behind the painted surface

vapor on the painted surface

2 Blooming: Formation of dull patches on the finished surface

Due to bad ventilation and defective paint

Allowing ventilation and using good quality paint

3 Fading: Gradual loss of colour

Due to the direct sunlight effect

Avoiding direct sunlight and painting colours of light shades

4 Flaking: Losing small portion of the paint surface

It is caused when the adhesion is poor

Painting on the surface which is having sufficient adhesiveness and allowing it to dry.

5 Flashing: Glossy patches on the painted surface

Poor workmanship and poor quality paint

Good quality of paint is applied with an efficient painter

6 Grinning: Clear visibility of the background

Final coat of the paint does not have sufficient opacity

Final coat should be given with utmost care.

7 Running: Paints run back and leaves small areas of surface uncovered

When the surface to be painted is smooth

The surface to be painted should be rubbed with emery paper and painting is carried out

8 Wrinkling The horizontal surface is too thickly painted

Thin coat of paint is applied on the horizontal surface

Defects in Painting

VarnishesVarnish is a solution of resins or resinous substances like shellac, cobalt in oil, turpentine or alcohol.



Necessityi. To provide a brilliant and decorative covering on wooden surface.ii. To protect the painted surface form the atmospheric action.iii. To protect unpainted wooden surface of doors, windows, floors, roof trusses etc.,

from atmospheric action.

Varnish being applied on wood surface Varnished wood surface

DistempersThese are paints used for the treatment of masonry walls. In distempers water is used instead of oil as carrier. These are also known as water paints. It is made by mixing whiting and glue boiled in water. These are mixed with water at the time of use and applied on the plastered surfaces. They give better finish and are long lasting. They are suitable for high-class interior decorative finishing work on the walls and ceilings. As distempers are affected by weather and peels off, if washed, so its use is restricted only to interior works.


plastering and painting

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