In this article we will discuss about:- 1. Objects of Painting 2. Characteristics of an Ideal Paint 3. Ingredients 4. Notes for Guidance 5. Painting on Different Surfaces 6. Failure 7. Defects.
Objects of Painting:
Following are the objects of painting a surface:
(i) It protects the surface from weathering effects of the atmosphere and actions by other liquids, fumes and gases.
(ii) It prevents decay of wood and corrosion in metal.
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(iii) It is used to give good appearance to the surface. The decorative effects may be created by painting and the surface becomes hygienically good, clean, colourful and attractive.
(iv) It provides a smooth surface for easy cleaning.
Characteristics of an Ideal Paint:
Following are the characteristics of an ideal paint:
(i) It should possess a good spreading power i.e. maximum area of the surface should be covered by minimum quantity of the paint.
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(ii) The paint should be fairly cheap and economical.
(iii) The paint should be such that it can be easily and freely applied on the surface.
(iv) The paint should be such that it dries in reasonable time and not too rapidly.
(v) The paint should be such that its colour is maintained for a long time.
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(vi) The paint should form a hard and durable surface.
(vii) The paint should not affect health of workers during its application.
(viii) The paint should not be affected by weathering actions of the atmosphere.
(ix) The paint should possess attractive and pleasing appearance.
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(x) The surface coated with paint should not show cracks when the paint dries.
(xi) When applied on the surface, the paint should form a thin film of uniform nature.
Ingredients of Oil Borne Paint:
An oil paint essentially consists of the following ingredients:
(1) A base,
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(2) A vehicle or carrier,
(3) A drier,
(4) A colouring pigment, and
(5) A solvent.
(1) Bases:
Table 15-2 gives the list of bases which are commonly used for paints. A base is a solid substance in a fine state of division and it forms the bulk of 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.
(2) Vehicles:
Table 15-3 gives the list of vehicles which are commonly employed in preparing paint.
The vehicles are the liquid substances which hold the ingredients of a paint in liquid suspension.
They are required mainly for two reasons:
(i) To make it possible to spread the paint evenly and uniformly on the surface in the form of a thin layer; and
(ii) To provide a binder for the ingredients of a paint so that they may stick or adhere to the surface.
(3) Driers:
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 other volatile liquid. The latter driers are compounds of the same metal.
But they are mixed with inert fillers such as barytes, whiting, etc. and then ground in linseed oil. The inert fillers serve as adulterants and the weight of inert filler in paint should not exceed one-fourth the weight of base.
They are used for the following purposes:
(i) To bring down the cost of paint;
(ii) To improve the durability of paint;
(iii) To modify the weight of paint; and
(iv) To prevent shrinkage and cracking.
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 litres 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 discolouration of a lead drier. It is required to take extreme care in mixing sulphate of manganese. Otherwise the spots will be formed on the painted surface.
Cobalt is the most reactive of drier metals and is generally regarded as a surface drier. It is widely used as the only additive in thin-film paint formulations. Lead is less reactive than cobalt has increasing restrictions in its applications because of ecological requirements.
Numerous other metals, including cerium and vanadium, have been used occasionally and are effective driers. Certain organic compounds also catalyze the drying of oils and have been used for this purpose when freedom from all metallic contamination is required.
Following precautions should be taken while using the driers:
(i) A drier should not be added until the paint is about to be used.
(ii) More than one drier should not be used in a mixture.
(iii) The driers need not be used with pigments that dry well.
(iv) The driers should not be used unnecessarily nor in excess especially in the finishing coat as they have a tendency to injure the colour of paint and to destroy the elasticity of paint and to cause flaking of paint.
(4) Colouring Pigments:
When it is desired to have a different colour than the base of a paint, a colouring pigment is to be added. The pigments are available in the form of fine powders in various colours and qualities.
Following are five divisions of the colouring pigments:
(i) Natural earth colours such as ochres, umbers, iron oxides, etc.
(ii) Calcined colours such as lamp black, Indian red, carbon black, red lead, etc.
(iii) Precipitates such as prussian blue, chrome green, chrome yellow, etc.
(iv) Lakes prepared by discolouring barytes or china clay with the help of suitable dyes.
(v) Metal powders such as aluminium powder, bronze powder, copper powder, zinc powder, etc.
Table 15-4 shows the colouring pigments which are used to create a particular tint of a paint.
(5) Solvents:
The function of a solvent is to make the paint thin so that it can be easily applied on the surface. It also helps the paint in penetrating through the porous surfaces. The most commonly used solvent is the spirit of turpentine.
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 colours 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 is often adultered with mineral oils and some of them have higher penetrating values but are otherwise inferior. The benzine 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:
(i) On evaporation, it should not leave any residue.
(ii) The paper coated with turpentine and left to dry should remain unstained and should then take ink freely.
(iii) When shaken vigorously, it should not froth i.e. form a mass of bubbles.
(iv) When warmed gently, it should not smell of resin or coal tar.
Notes for Guidance in the Process of Painting:
Some of the important points to be noted before we take up the discussion of process of painting are mentioned below:
(1) Brushes:
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 this respect, they can be distinguished from horse hairs.
It is preferable to use a round brush in painting. 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 dry 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.
(2) Paints:
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 a long duration, the solidification of the base and the pigment occurs.
The procedure for preparing paint from stiff paint is as follows:
(i) The sufficient stiff paint is taken in a pot. The remaining stiff paint is to be covered or left with a layer of linseed oil.
(ii) The linseed oil which may either be raw or boiled or pale is then mixed with stiff paint by a stick.
(iii) The other ingredients of paint are then added.
(iv) If a colour is required, suitable pigment is added and thoroughly mixed.
(v) A second pot is taken and it is covered with a canvass which is tied tightly.
(vi) The mixed paint is then allowed to pass through the canvass of second pot. The brushes may be used for this purpose.
(vii) The paint is then ready for use. The mixed paint may be prevented from drying by maintaining a thin film of linseed oil or of water at top.
(3) Knotting:
The term knotting is used to indicate the covering or killing of all knots in woodwork with a substance through which the resin cannot exude or come out.
There are three methods of knotting as mentioned below:
(i) Ordinary or Size Knotting:
This is applied in two coats. For the first coat, the red lead ground in water and mixed with strong glue size is used in hot condition. This coat dries in about ten minutes and then second coat is applied. The second coat consists of red lead ground in oil and it is thinned by boiled oil and turpentine.
(ii) Patent Knotting:
This is applied in two coats. For both the coats, the varnish prepared by dissolving shellac in methylated spirits of wine is used.
(iii) Lime Knotting:
The knot is covered by hot lime and it is left for 24 hours. The surface is then scrapped off and then ordinary knotting is carried out.
(4) Stopping:
The term stopping is used to indicate the rubbing down of the surface after the first coat of paint is applied. The rubbing is done by means of pumice-stone or glass-paper or both. Before rubbing is commenced, the holes, cracks, etc. on the surface are filled with ordinary putty made from whiting and linseed oil. The putty becomes hard when it dries.
The term hard stopping is used when instead of ordinary putty, an admixture of one-third white lead and two-third ordinary putty, is filled in holes, cracks, etc. It is adopted for superior work.
(5) Coats:
The paint is usually applied in three or four coats. The first coat is known as the priming coat, the second one as under coat and the remaining as finishing coats.
The priming coat creates a layer or film which provides adhesion of the paint with the surface. It 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 surface are removed by this coat.
The finishing coat or coats are then applied as per requirements.
(6) Spray Painting:
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.
Following are the advantages of spray painting:
(i) The speed of work increases considerably.
(ii) An experienced painter can create artistic patterns or designs on the surface by spraying paints of different shades.
(iii) It is found to be economical in material and labour. The two coats of spray would suffice whereas ordinary brush painting would require three or four coats.
(iv) The mechanical equipment is such that the paint remains always in a state of motion. This ensures uniform mixing of ingredients of the paint.
Painting on Different Surfaces:
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 Woodwork:
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 in 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 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 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 Ironwork and Steelwork:
The surface of iron or steel to receive the paint should be free from rust, grease, dirt, etc. The suitable equipment such as wire-brushes, 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 Steelwork:
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 Ironwork:
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 0.13 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 heating 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 postpone painting till such spots cease to appear.
(v) If there are chances for the discolouration of the painted surface, it is necessary to clean the surface and to sterilise it with an antiseptic wash. The organisms responsible for discolouration 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 surfaces. 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 surfaces. 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.
Failure of Paint:
The failure of paint job on wall or wooden members or structural steelwork is not uncommon. The paint job, though 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 paint.
Following are the main causes of failure of a paint job:
(1) Bad Workmanship:
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.
(2) Conditions for Painting:
The job of painting should be carried out under favourable 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.
(3) Moisture:
The leakage through sanitary installations, floors, roofs, water pipes, etc., make the painted surface moist and in case of newly constructed building, the water used during construction activity may require time to evaporate. Whatever may be the reason, the presence of moisture accelerates the process of separating the paint layer from surface.
(4) Salts and Alkalies:
The movement of moisture can transport salts from either internal volume of masonry or new deposits. Such salts and alkalies saponify the oil paints.
(5) Unsuitable Surfaces:
If the surfaces are not properly prepared or treated to receive the paint, it may lead to the failure of paint job.
(6) Wrong Choice of Paint:
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.
Defects in Painting:
Following are the usual defects which are found in the painting work:
(1) Blistering:
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.
(2) Bloom:
In this defect, the formation of dull patches occurs on the finished polished surface. It is due to the defect in paint or bad ventilation.
(3) Fading:
The gradual loss of colour is known as the fading and it is mainly due to the effect of sunlight on pigments of paint.
(4) Flaking:
A small portion of the painted surface is sometimes seen loose. It is known as the flaking and is due to poor adhesion.
(5) Flashing:
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.
(6) Grinning:
When the final coat of paint has not sufficient opacity, the background is clearly seen. This is known as the grinning.
(7) Running:
The paint runs back and leaves small areas of surface uncovered. This defect occurs when the surface to be painted is too smooth.
(8) Sagging:
When a vertical or inclined surface is too thickly painted, the defect of sagging occurs.
(9) Saponification:
The formation of soap patches on the painted surface is termed as the saponification and it is due to chemical action of alkalies.
(10) Wrinkling:
When a horizontal surface is too thickly painted, the defect of wrinkling becomes prominent.