SANDEEP SINGH, KUNAL SONI, IRSE 2009 (P)
According to the nature of the surface and the finishing required, the final finishing of all surfaces such as walls, ceilings, woodwork and metal work,the surface should be coated with paint or varnish or distemper, etc.
The paints are coatings of fluid materials and they are applied over the surfaces of timber and metals. The varnishes are transparent or nearly transparent solutions of resinous materials and they are applied over the painted surfaces. The distempers are applied over the plastered surfaces. The protective power granted by the application of paints, varnishes and distempers decreases with the passage of tome and hence they are to be periodically applied at the intervals of about 1 to 5 years.
The fresh lime is slaked at site of work and mixed thoroughly with sufficient quantity of water in a tub. It is then screened through a clean cloth. The clan gum dissolved in hot water is then added at the rate of 20 N per m3 of lime. The rice may be used in place of gum. The surface to be whitewashed should be cleaned before the work is started. For whitewashing walls which are whitewashed before, the old loose whitewash is to be first removed and repairing to the plaster is carried out, if necessary. The lime is toxic for germs. It reflects light and thus it increases the brightness of the surface. The whitewashing therefore is extensively used for interior wall surfaces and ceilings of houses. The process of whitewashing is sometimes used for exterior wall surfaces also. A satisfactory work gives an opaque smooth surface with uniform white color and does not readily come off on the hand, when rubbed.
This is prepared by adding the coloring pigment to the screened whitewash. It should be seen that the coloring pigment is not affected by the presence of lime. Ordinarily, the yellow earth is popular of colorwahsing. Generally, the walls are colorwashed and ceilings are whitewashed. The mixture is to be kept constantly stirred during use. The colorwash is applied in the same fashion as the whitewash. A satisfactory work does not give out powder when the finished surface is rubbed with the fingers. The process of colorwashing imparts cleanliness and pleasant appearance of the surfaces which are treated.
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 a present a neat appearance. They are available in a variety of colors.
A distemper is composed of base, carrier, coloring pigments and size. For base, the whiting or chalk is used and for carrier, the water is used. Thus it is more or less a paint in which whiting or chalk is used as base instead of whit lead and the water is used as carrier instead of linseed oil. The distempers are available I powder form or paste form. They are to be mixed with hot water before use. The oil-bound distempers are a variety of an oil paint in which the drying oil is so treated that it mixes with water. The emulsifying agent which is commonly used is glue or casein. As the water dries, the oil makes a hard surface which is washable. It should be remembered that most of the manufacturers of ready made distempers supply completely directions for use of their products. These directions are to be strictly followed to achieve good results.
Following are the objects of paintings a surface:
The brief descriptions of different types of paints are given below.
The very finely ground aluminium is suspended in either quick-drying spirit varnish or slow-drying oil varnish as per requirement. The spirit or oil evaporates and a thin metallic film of aluminium is formed on the surface. The advantages of an aluminium paint are as follows:
This paint essentially consists of oil and a strong drier. A pigment such as chromium oxide or lead or red lead or zinc chrome is taken and after mixing it with some quantity of very fine sand, it is added to the paint. The advantages of as anticorrosive paint are as follows:
This is a peculiar type of paint and it is applied on the surfaces which are exposed to the acidic gases and steam.
this paint is prepared by dissolving asphalt or mineral pitches or vegetable bitumen I any type of oil or petroleum. A variety of bituminous paints is available. The paint presents a black appearance and it is used for painting iron.
Following are the advantages of cement paints:
An oil paint essential consists of the following ingredients:
A base is a solid substance in a fine state of division and it forms the bulk of a paint. It determines the character of the paint and imparts durability to the surface which is painted. It reduces shrinkage cracks formed on drying and it also forms an opaque layer to obscure the surface of material to be painted.
The vehicles are the liquid substances which hold the ingredients of a paint in liquid suspension. They are required mainly for two reasons:
These substances accelerate the process of drying. A drier absorbs oxygen from the air and transfers it to the linseed oil, which in turn, gets hardened.The various patented driers are available in the market. They may be either in the form of soluble driers or paste driers. The former driers are compounds of metals such as cobalt, lead, manganese, etc. Dissolved in linseed oil or some to her volatile liquid. The latter driers are compounds of the same metal. But they are mixed with inert fillers such as barites, whiting, etc. and then weight of inert filler in a paint should be not exceed one-fourth the weight or base. They are used for the following purposes:
The litharge, red lead and sulphate of manganese can also be used as driers. The litharge is the most commonly used drier, the proportion being 1.25 N to 5 liters of oil. The red lead is less effective than litharge and it is to be used when its addition does not interfere with the tint of the paint. The sulphate of manganese is used with zinc paints so as to eliminate the risk of discolorate The turpentine is inflammable, evaporates rapidly and dries the oil consequently. The use of a thinner in paint reduces the protective value of the coating, flattens colour and lessens the gloss of the linseed oil as the spirits evaporate leaving an excess of colour not mixed with the oil. The turpentine is a transparent volatile liquid and it is obtained by distilling the resinous exudation of some varieties of pine trees. It has a pungent odour and it often adultered with mineral oils and some of them have higher penetrating values but are otherwise inferior. The benzene and naphtha are used as substitutes.
A solvent or thinner is not generally used in finishing coats on the exposed surfaces as it has a tendency to impair or damage or injure the firmness of the paint. But if the surface is to be exposed to the sun, the turpentine is added to reduce the possibility of the paint blistering. Following are the simple tests for ascertaining the purity of turpentine:
Some of the important points to be noted before we take up the discussion of process of painting are mentioned below:
It is necessary to have good brushes for painting. The brushes should be composed of bristles and not of horse hairs. The bristle brushes are elastic and possess good paint-holding capacity. The bristles are split at ends and in the respect, they can be distinguished from horse hairs. It is preferable to use a round brush in printing. The new brushes should be soaked in water upto level of bristles for a period of about two hours and then they should be allowed to try for a period of about one hour. During painting, the brush should be immersed in paint upto about one-third length of the bristle and the excess paint in the brush is removed by gently pressing the bristles against the inside surface of the pot. After the work is over, the brushes should be cleaned at once with kerosene oil.
The ready-mixed paints of different make and various brand are available in the market. They are available in different tints and can be applied in the same form as received. The ready-mixed paints are normally expensive and they are to be used soon after opening the sealed container because of the fact that volatisation of the vehicle and solvent will take place when exposed to the atmospheric oxygen. If the ready-mixed paint is kept exposed to air for along duration, the sclidification of the base and the pigment occurs. The procedure for preparing paint from stiff paint is as follows:
The priming coat creates a layer or film which provides adhesion of the paint with the surface. It is also protects the surface from weathering actions. The suitable material for priming coat should be used, depending on the nature of surface to be painted. The under coat serves to provide foundation or support to the finishing coat. The surface is made even and all irregularities of the surfaces are removed by this coat. The finishing coat or coats are then applied as per requirements.
Instead of the ordinary brushes, a spraying pistol may be used for painting work. The pistol works under compressed air and the paint thrown through the pistol on the surface forms a thin uniform film or layer of paint on the surface. The spray painting is superior to painting by brushes.
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:
Normally four coats of paint are required for new woodwork. The process of painting is carried out as follows:
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 sand-paper or fine pumice stone. The old paint can also be removed by applying any one of the following two paint solvents:
As the paint will not adhere to their 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 to galvanized iron work 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 tow solutions is applied on the surface:
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. 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.
Following are the usual defects which are found in the painting work:
This defect is caused by the water vapour which is trapped behind the painted surface. The formation of bubbles under the film of paint occurs in this defect. It may occur from various causes such as imperfect seasoning of timber, excess oil used in final coat, etc.
In this defect, the formation of dull patches occurs on the finished polished surface. It is due to the defect I paint or bad ventilation.
The gradual loss of colour is known as the fading and it is mainly due to the effect of sunlight on pigments of paint.
A small portion of the painted surface is sometimes seen loose. It is known as the taking and is due to poor adhesion.
Sometimes the glossy patches are seen on the painted surface. This is known as the flashing and it is mainly due to poor workmanship, cheap paint or weather actions
When the final coat of paint has not sufficient opacity, the background is clearly seen. This is known as the grinning.
The paint runs back and leaves small areas of surface uncovered. This defect occurs when the surface to be painted is too smooth.
when a vertical or inclined surface is too thickly painted, the defect of sagging occurs.
The failure of paint job on wall or wooden members or structural steel work is not uncommon. The paint job, through an easy job, has peculiarities of its own. At the same time, a failed paint job involves substantial expenditure by way of scraping the old paint, repairing the surface and cost of new pant. Following are the main cause of failure of a paint job:
The paint job demands a certain degree of skill. A careless painter is in the habit of thinning paint too much so that he can save material and labour both. The layer of paint can even be absent for portions of surface which are difficult to reach.
The job of painting should be carried out under favorable conditions only. It should be seen that dirt, dust and moisture do not get entrapped during the process of painting. Also, very high or low temperatures and humidity during the application of paint can also seriously affect the performance of most of the paints.
The leakage through sanitary installations, floors, roofs, water pipers, etc., make the painted surface moist and in case of newly constructed building, the water used during construction activity any require time t evaporate. Whatever may be the reason, the presence of moisture accelerates the process of separating the paint layer from surface.
Depending upon the climatic conditions, nature of surface to be painted and various other factors affecting the performance of paint, the choice of paint should be made. It is found that low quality paints are cheap in initial cost. But the durability of such painted surfaces is very poor.