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Painting on different surfaces, woodwork, Metal etc.

Er. Parbhakar Dwivedi
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The process of painting depends on the nature of the surface to be painted. A brief description of painting on each of the various surfaces is given below:

(1) New wood work: Normally four coats of paint are required for new woodwork. The process of painting is carried out as follows:

(i) The surface of 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 woodwork is thoroughly cleaned and the heads of nails are punched to a depth of 3 mm below the surface.

(ii) The surface of the woodwork is then knotted.

(iii) The priming coat is then applied on the surface of new woodwork. Generally, the priming coat is applied before the woodwork is placed position.

(iv) The process of stopping is then carried out.

(v) The subsequent coats of paint, namely, under coats and finishing coats, are then applied on the surface. The extreme care should be taken to see that the finishing coat presents smooth and even surface and that no brush marks are seen on the finished work.

(2) Repainting old wood work: If the paint on the old woodwork has cracked or has developed blisters, it is to be removed. If the surface has become it should be cleaned by rubbing down sand-paper or fine pumice stone.

The old paint can also be removed by applying any one of the following three paint solvents:

(i) A solution containing 2 N of caustic soda to a litre of water is prepared and used to wash the surface. The paint dissolves and the surface becomes clean.

(ii) A mixture consisting of one part of soft soap and two parts of potash is prepared and one part of quicklime is then 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.

(iii) 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 old paint from the surface, the woodwork is painted as in case of painting on new woodwork.

(3) New iron work and steel work: The surface of iron or steel to receive the paint should be free from rust, grease, dirt etc. The suitable equipment such as wire-bruises, scrapers, etc. are used to remove all loose scales, marks, etc. from the surface. The 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 finishing coat should present a smooth finish and precaution should be taken to avoid the presence of brush marks on the final painted surface.

(4) Repainting old ironwork and steel work: The old surface should be thoroughly cleaned by the application of soap-water and if grease is present, it should be removed by washing the surface with lime and water. If it is necessary to remove old paint, the surface should be burnt, usually by a blow lamp and then old paint should be scraped off or dissolved and removed by using any paint solvent. After the surface is thus prepared, the painting is carried out as in case of new ironwork or steelwork.

(5) Galvanized iron work: As the paint will not adhere to the surface of galvanized ironwork, some treatment is to be given to the surface before a priming coat is applied. It is a general rule not to paint the galvanized ironwork until it has been exposed to weather for a period of one year or so. However, if it is necessary to paint new galvanized ironwork, any one of the following two solutions is applied on the surface:

(i) A solution containing 0.40 N of copper acetate to a litre of water.

(ii) A solution containing 0.18 N each of muriatic acid, copper chloride copper nitrate and sal-ammoniac to a litre of soft water. This much quantity of solution will cover an area of about 250 to 300 m2.

The solution is taken in a glass vessel or earthenware vessel. This will prevent the precipitation of copper salts. When the solution is applied on the galvanized ironwork, the surface is turned black and after a period of about 12 hours, the coat of paint may be applied on the surface.

Alternatively, a wash of washing soda or zinc sulphate may be given on the surface and when it dries, a priming coat of red lead mixed with linseed oil and turpentine may be applied on the surface. When priming coat dries, a suitable paint may be applied on the surface.

(6) Metals: The surface of the metal to be painted should be clean and free from dirt, grease, etc. It should be such as to provide key for the paint. Depending upon the nature of metal, suitable paint is selected. For instance, the priming coat for aluminium surface should be of zinc chromate and that for zinc surface, it should be of zinc oxide.

(7) Plastered surfaces: For successful application of paint on cement plastered surfaces, the following factors should be carefully considered:

(i) There is considerable amount of moisture in newly constructed wall with cement plaster. It is necessary to allow the moisture content to escape. The actual time required for drying of wall surface will depend on atmospheric conditions and ventilation. But it usually requires about 3 to 6 months for the surface to be ready to receive paint. The artificial beating may in some cases be employed in combination with ventilation.

(ii) The cement plaster is alkaline in nature because lime is liberated during the hydration of cement. Hence the oil based paints and distempers are liable for alkali attack, especially in presence of moisture. Hence the coats of alkali resistant primer paints should be applied or paints not containing any oil should be used.

(iii) The defects in cement plastered surfaces develop due to various reasons. Such defects should be removed and the surface should be prepared to receive the paint.

(iv) The spots showing efflorescence should be brushed off and the surface should be watched for few days. If spots of efflorescence develop again the process of brushing off should be repeated. It is desirable to postpose painting till such spots cease to appear.

(v) If there are chances for the discoloration of the painted surface, it is necessary to clean the surface and to sterilise it with an antiseptic wash. The organisms responsible for discoloration are algae, mould, lichen, etc. They may be black, green purple or red in colour. The conditions favourable for their easy and free growth are dampness and high humidity.

With the above remarks in view, the usual paints recommended for cement plastered surfaces are cement paints, emulsion paints, oil paints, silicate paints etc.

(8) Concrete surfaces: The cement paint is used to paint the concrete surface. The paint is available in a powder form and it is dissolved in water to a workable consistency. The paint thus prepared should be consumed within 2 to 3 hours. The two coats at an interval of 24 hours are just enough. The water should be sprayed at intervals to provide curing to the painted surface.

(9) Floor surfaces: The enamels are used for painting floor surface the selected enamel should be strong enough to resist abrasion, moisture and alkali action. It should also be of shining nature and quick drying type.

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