Chapter 22
Painting
Paints are coatings of fluid materials which are applied as a final finish to surfaces like walls, ceiling, wood and metal works.
Painting is done to protect the surface from the effects of weathering, to prevent wood from decay and metal from corrosion, to provide a decorative finish and to obtain a clean, hygienic and healthy living atmosphere.
22.1 TYPES OF PAINTS
- Enamel paints: These paints are available in numerous shades. They mainly consist of white lead or zinc white, resinous matter and petroleum spirit. Their formation into hard, impervious, decay-resistant enamel-like surface soon after application protects it from being affected by acids, alkalies, fumes and gas, hot and cold water, etc. They can be used for internal as well as external purposes.
- Cement paints: These include a variety of paints in which cement is the main constituent responsible for the hardness and durability of the painted surface. They are available in dry, powder form. They are waterproof. It is desirable to provide cement paint on a rough surface rather than a smooth surface because its adhesion power is more on rough surface than on smooth surface. They prove to be economical as compared to oil paints. They are suitable for painting fresh plasters having high alkalinity because cement paints are not likely to be attacked by the alkalinity of the masonry surface. It is not necessary to remove the existing paint for the application of new paint.
- Oil paints: They are generally applied in three different layers with varying composition. These are termed as primes, undercoats and finishing coats. The dampness of the wall affects the life of the oil paint, hence it must not be applied during damp weather.
- Cellulose paints: They are prepared from nitro cotton, celluloid sheets, photographic films, etc. The cellulose paints harden by evaporation of thinning agents. The surface painted with cellulose can be washed and cleaned easily. They are little costlier than other paints.
- Aluminium paints: Finely ground aluminium is suspended in either quick-drying spirit varnish or slow-drying oil varnish as per requirement. As the spirit or oil evaporates, a thin film of aluminium is formed on the surface. This paint forms a better protective surface over steel and iron. They are impervious to moisture and possess high electrical resistance. They have a good appearance and are visible in darkness.
- Emulsion paints: These paints contain polyvinyl acetate, synthetic resins, etc. They are easy to apply and are retained for a long period and can be cleaned easily with water. For a rough plastered surface, a thin coat of cement paint may be first applied to smoothen the surface. It is necessary to have a sound surface to receive the paint.
- Anticorrosive paints: These consist of oil and a strong drier. The pigments such as chromium oxide, lead or zinc chrome is taken and after mixing it with a small quantity of very fine sand it is added to the paint. These are cheap and last for a long duration. They are black in colour.
- Synthetic rubber paints: These paints are prepared from resins. They have an excellent chemical resistant property. They can be applied to surfaces that may not be completely dry. They offer good resistance to water and are not affected by heavy rains. They dry very quickly. They are not affected by weather and sunlight and are quite easy to apply.
22.2 USES OF PAINT
- It protects the surface from weathering effects of the atmosphere and the actions by other liquids, fumes and gases.
- It prevents decay and corrosion in metal.
- It is used to give good appearance to the surface.
- It provides a smooth surface.
22.2.1 Characteristics of a good paint
A good paint should possess high spreading power and should be durable, tough and resistant to wear on drying. It should work smoothly and freely and should not crack, fade or change colour. Its surface should become dry in 9 hours and hard enough to take another coat in 24 hours. We should be able to spread it into a very thin layer and it should provide a smooth and pleasing appearance.
22.3 PAINTING OF DIFFERENT SURFACES
22.3.1 New woodwork
Normally, four coats of paint are required for new woodwork. The process of painting is as follows:
- The surface of the woodwork is prepared to receive the paint for satisfactory working. It is necessary that the woodwork is sufficiently seasoned and it does not contain more than 15 per cent moisture at the time of painting. The surface of the woodwork is thoroughly cleaned and the heads of nails are punched to a depth of 3 mm below the surface.
- The surface of the woodwork is then knotted.
- The process of cleaning the surface using sand paper is then carried out.
- The subsequent coats of paint, namely undercoat and finishing coats, are then applied on the surface. Extreme care should be taken to see that the finishing coat presents a smooth and even surface and that no brush marks are seen on the finished surface.
22.3.2 Repainting old woodwork
If the paint on the old woodwork has cracked or has developed blisters, it is to be removed. If the surface has become greasy, it should be cleaned by rubbing down with sandpaper or fine pumice stone. The old paint can also be removed by applying any one of the following three paint solvents.
- A solution containing 200 g of caustic soda in a litre of water is prepared and used to wash the surface. The paint dissolves and the surface becomes clean.
- A mixture consisting of one part of soft soda and two parts of potash is prepared and one part of quick lime is added afterwards. This mixture is applied on the surface in a hot state and allowed to stay for about 24 hours. The surface is then washed with hot water.
- A mixture consisting of equal parts of washing soda and quicklime is brought to a paste form by adding required quantity of water. It is applied on the surface and kept for about an hour. The surface is then washed with water.
After removing the old paint from the surface, painting is carried out in 2 or 3 coats of an oil paint.
22.3.3 Painting new iron work and steel work
The surface of iron or steel to receive the paint should be free from rust, grease, dirt, etc. Suitable equipments such as wire brushes and scrapers are used to remove all loose scales, marks, etc. from the surface. Water with caustic soda or lime is used to remove grease. The cleaned surface is provided with a film of phosphoric acid. This film protects the surface from rust and it also facilitates the adhesion of paint. The coats of paint are then applied. The paint suitable to iron and steel surfaces should be selected for each coat. The priming coat or first coat is made by dissolving fine red lead powder in boiled linseed oil. After this coat has dried, two or more coats of the desired paint are applied over the priming coat, with the help of a brush or spray gun The finishing coat should present a smooth finish and precaution should be taken to avoid the presence of brush marks on the final painted surface.
22.4 WHITE WASHING
The process of white washing process can be done through carried out under the following operations:
22.4.1 Preparation of white wash
The white wash is prepared from fresh burnt shell or pure stone lime mixed with water. Shell lime is preferred to pure lime as it is whiter and slakes more perfectly to a smoother paste.
To prepare white wash, fresh lime is slaked at the site of work and is dissolved in a tub with sufficient quantity of water. After slaking, it is allowed to remain in the tub of water for 2 days and then stirred up with a pole until it attains the consistency of thin cream. This mixture is then strained or screened through a clean coarse cloth. Clean gum dissolved in hot water is then added at the rate of 2 kg/m3 of lime to the white-wash water. The solution so formed is called as white wash. To prevent the glaring effect due to whitewash, sometimes copper sulphate at the rate of 4 kg/m3 of thin cream is added. In order to have better adhesive properties, alum or common salt may be added in the same proportion as gum.
22.4.2 Preparation of surface
Before applying white wash to a new wall surface, it is essential that the surface should be cleaned, brushed and made free from loose materials and any other foreign matter. If the surface to be coated is oversmooth, then the coats will not stick to it. In such cases, the surface should be rubbed with sand paper to ensure proper adhesion of white wash.
In case of re-white washing, all loose materials and scales should be scrapped off. The old loose white wash is removed by rubbing with sand paper. All holes on the wall, irregularities of surface and minor repairs are corrected in advance by filling with lime putty.
All greasy spots should be given a coat of a mixture of rice water and sand so that the finishing wash may stick to the surface. If old white wash is discoloured by smoke or other reasons as in kitchens, factories, restaurants, etc., then in such cases the surfaces should be given a wash of a mixture of wood ashes and wastes or yellow earth, before the application of white wash. Cement plastered walls should be washed with a weak solution of soap and dried before applying white wash.
22.4.3 Application of white wash
The white wash is applied to a specified number of coats with a brush. Usually, three coats are required for new work and scrapped surfaces, while one or two coats are considered sufficient for old work. For each coat, one stroke is given from the top downwards and the other from the bottom upwards over the first stroke, and similarly one stroke from the right and another from the left over the first brush before it dries. Each coat should be allowed to dry before applying the next coat. The finished dry surface should not show any signs of cracking or peeling and should not come off readily on fingers when rubbed.
22.5 COLOUR WASHING
A colour wash is usually prepared by adding the necessary colouring pigments in suitable quantities to the screened white wash or liquid mixture of white wash. It should be ensured that the colouring pigment is not affected by the presence of lime. The colour wash is applied in exactly the similar manner as white wash. The colour is constantly stirred with a stick during its use. The quantity of colour wash, which is just enough for the day's work, should be prepared at a time in the morning.
- Normally, the ceilings are white washed and the walls are colour washed.
- Before applying colour wash on new surfaces or scrapped surfaces, a coat of white wash should be applied. This coat will act as a priming coat and incidentally result in economy also.
- In the case of old work, a coat of colour wash is first applied over the patches or repaired work and rectified surface spots. Two or more coats of colour are then applied on the entire surfaces till it presents the desired finish.
- A satisfactory finish should not give out the powder when rubbed with fingers.
22.6 DISTEMPERING
The main object of applying distemper to the plastered surfaces is to create a smooth surface. The distempers are available in the market under different trade names. They are cheaper than paints and varnishes and they present a neat appearance. They are available in a variety of colours.
22.6.1 Properties of distempers
- On drying, the film of distemper shrinks. Hence, it leads to cracking and flaking, if the surface to distemper is weak.
- The coating of distemper is usually thick and they are more brittle than other types of water paints.
- The film developed by distemper is porous in character and it allows water vapour to pass through it. Hence, it permits new walls to dry out without damaging the distemper film.
- They are generally light in colour and they provide a good reflective coating.
- They are less durable than oil paints.
- They are treated as the water paints and they are easy to apply.
- They can be applied on brickwork, cement plastered surface, insulating board, etc.
- They exhibit poor workability.
- They prove to be unsatisfactory in damp locations such as kitchens and bathrooms.
22.6.2 Process of distempering
The application of distemper is carried out in the following manner:
22.6.2.1 Preparation of surface
The surface to receive the distemper is thoroughly rubbed and cleaned. The important points to be noted are:
- The new plastered surfaces are to be kept exposed for a period of 2 months or so to dry out before the distemper is applied on them. The presence of dampness on the surface results in failure of the distemper coating.
- The surface to receive the distemper should be free from any efflorescence patches. The patches are to be wiped out by a clean cloth.
- The irregularities of the surface such as cracks and holes are to be filled by lime putty or gypsum and are allowed to become hard before distemper is applied on the surface.
- If distemper is to be applied on the existing distempered surface, the old distemper should be removed by profuse watering.
22.6.2.2 Priming coat
After preparing the surface to receive the coats of distemper, a priming coat is applied and it is allowed to become dry. For readymade distempers, the priming coat should be composed of materials as recommended by the makers of distempers. For locally made distempers, the milk can be used for priming coat. One litre milk will cover about 10 m2 of the surface.
22.6.2.3 Coats of distemper
The first coat of distemper is then applied on the surface. It should be of a light tint and applied with great care. The second coat of distemper is applied after the first coat has dried and become hard.
- The distempering should be done in dry weather to achieve better results.
- The oil-bound distemper or washable distemper adheres well to oil-painted walls, woods, corrugated iron, etc. But a priming coat of pure milk should be applied before distempering is done on such surfaces.
- The application of distemper by a spraying pistol is superior to that by brushes. Spraying affords a smooth and durable film of distemper.
22.7 TILING OF WALLS
Nowadays walls of the bathrooms, kitchens, work area, etc. are given a covering of glazed tiles in order to provide improved cleanliness and aesthetic appearance. Tiles are available in various forms, shapes, sizes and colours.
Before fixing the tiles, the verticality of all the corners should be checked and all the concealed plumbing and electric works should be completed and checked for their leak proofness. The plastered surface should then be given a final bedding of cement paste 1:2 with very fine sand. Tiles should be kept soaked in clean fresh water for 30 minutes before mixing. Cement mix should evenly be applied on the backside of the tiles and fixed in position by tapping into the correct position. The joint should be kept minimum (1.5 mm). Then the corners should be stroked with a mallet to check whether there is any hollow sound, which indicates poor workmanship. The joints should then be cleaned and a finish of white or coloured cement should be given. Curing should be continued for at least 15 days.
REVIEW QUESTIONS
- Why is painting done on wooden surfaces?
- What are the types of paints mainly used?
- What are the main uses of paints?
- What are the main characteristics of a good paint?
- How is painting done on wooden surfaces?
- What is white washing and how is white wash prepared?
- What are the properties of distempers?
- Briefly discuss the process of distempering.
- What do you mean by tiling of walls?
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