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L.Srikumar Pai
B.Sc( Engg.), MIE, MIWWA, MICI
Civil Engineer & CAD Specialist
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See my 3d perspectives using AutoCAD & 3DS Max.
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Construction: Painting tips

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Wooden surfaces:

PU sealer, matt and glossy has to be mixed with the hardener in the ratio 80:20 of base and hardener. The mixture is to be allowed to mature for a minimum period of 30 minutes and should be used within 4 hours of mixing.

Melamine sealer, matt and glossy - has to be mixed with the hardener in the ratio of 90:10 of base and hardener. The mixture is to be allowed to mature for a minimum period of 30 minutes and should be used within 8 hours of mixing. Melamine should not be exposed to ammonia vapour at the time of mixing or application / drying.

Wood Stains can be used to give the required shade to the wooden surface. Wood stains can be mixed with Melamine, but not with TouchWood or PU wood sealer / finish. Wood stain is to be applied on raw wood before application of the clear coat. Or it is to be applied over the sealer coat and before the top coat. The wood stains are for interior usage only and will fade if applied on exterior wood. Normal Putty which is used to smoothen out walls cannot to be used on wood. The previous coat of finish has to be very thoroughly sanded for the new finish adhesion.

The choice of application can be affected by the type of finishes to be used. A finish formulated for compressed air spraying could well be suitable for compressed air hot-spraying or airless spraying by adjustment with thinner. Some types of finishes may be ill-suited to or impracticable with a certain method of application, for example, some catalysed finishes with "pot lives" of only few minutes would be a hazard in hot spraying application. A good finishing system depends on having the right equipment, materials and processing techniques, thus requiring co-operation between the equipment supplier, the paint manufacturer and the user.

Exterior walls

Horizontal surfaces like awnings, chajjas, terrace borders, etc. should be given one additional coat for better protection as these areas are prone to water accumulation.

No interior primer or putty or Plaster of Paris is to be applied under the exterior finishes. Doing so will affect the performance of the paint.

New surfaces must be coated with one additional coat of self priming (1:1 by volume) with water.

The cement coating has to be done within 2 hours of mixing with water. After the coating and after 4 hours of drying, curing with water to be done 2 - 3 times a day for one coat.

Interior walls

Darker shades may require an additional coat for proper hiding. The actual shade, especially for darker shades, can be observed only after the film is dry and not in the can or in the wet state.

Felt rolling is to be done only for Royale and Lustre finish. The darker shades are never to be felt rolled since this will cause foaming due to presence of more surfactants in them.

Solvent based paints (those using thinner other than water) should be given twice as long a drying time than given here in case of high humidity climate like monsoons.

Putty can be applied to make the substrate smoother; however it has to be sandwiched between 2 coats of primer.

Paint application

Brush Application

Brushing is the oldest method of application having versatility under unlimited variable conditions and situations. It is still the best method to paint large complex objects. Brushing helps to assist in the penetration of the paint. Wastage of the paint is relatively low. It is mainly used for decorative and maintenance application.

Choice of Brush
The best quality of brushes are made up of hog bristles. These bristles taper from the roof to the tip where each is split into two or more fine strands known as flag, which enable a high degree of finish.

In cheaper brushes a proportion of other animal hair or vegetable fibres are used. Brushes made up of nylon bristles are also available. These are superior to pure bristles in their resistance to wear, but are smooth and non-absorbent. Hence they carry less paint and have a tendency to run-off from the brush.

A brush of suitable size for the work at hand should always be selected. Using a small brush on a large area makes it difficult to apply an even coating and slows down rate of working; while using a large brush on a narrow area makes accurate "cutting in" impossible.

Use and Maintenance of Brushes
In a new brush, bristles hold dust and broken bristles must be washed out before using the brush for finish work. Soap water can be used for washing out the new brushes. A paint brush works best when it is 'broken in' that is, when it has acquired a level at the end of bristles. Brush must be 'washed in' in order to distribute the paint throughout the thickness of the brush. Brush is dipped in the paint and is rubbed against the sides of the kettle. After painting is over, excess paint is brushed back into the container, the brush is scrapped clean with a blunt putty knife, rubbed on a suitable surface, then rinsed out in thinner and 'spun' dry. It should be thoroughly washed in soap water and dried.

Brushing Technique

-At a time, the area to be painted has to be small, to facilitate the distribution of paint without loss of time. Otherwise, brush marks are inevitable.
-The pressure of the brush must be equal for all strokes so that the paint easily penetrates to the surface.
-In order to ensure the levelling and elimination of brush marks, "laying off" should be done with very less pressure.

Paints suitable for Brushing
Air drying type, based on long/medium oil length alkyds, emulsion, distemper are ideal for brush application. Quick drying type paints based on medium to short oil length alkyd and fast evaporating solvent like Xylene are not suitable.

Roller Application
Hand Rolling
At present hand rolling is done mainly for decorative and maintenance paintings. Roller Coating application is of particular value on broad plain surfaces. The covering material for the roller may be plastic sponge. When equipped with a long handle extension, roller may be used for floor painting and work otherwise inaccessible.

For large areas it is more convenient to work from a bucket than a tray; a perforated grid is placed inside the bucket, the roller being dipped into the paint and then rolled over the grid to remove surplus material and distribute it evenly. With the tray, a reservoir at an end holds the paint; after charging the roller, it is rolled out on the platform of the tray. In use, the roller is rolled over the surface in criss-cross strokes, working the material out evenly. Paints normally used for brushing can be hand-rolled.

Although not as quick as spraying, it is usually quicker than brush application, especially on rough surfaces. Another advantage is that, it enables the semi-skilled operator to obtain a reasonable standard of finish and for this reason, it is likely to be popular with amateur painters. Roller application on narrow or broken surface does not show any special advantage over brushing.

Machine Rolling
Roller coating application is used virtually exclusively for coating metal containers such as tins and also for some drums, kegs and barrels which can be formed from the coated sheet.

The coating material is fed on the rubber roller through two smaller feed rollers which are of precision ground iron or steel, and which are also adjustable for pressure in order to regulate the film thickness of the coating. The sheets pass between rollers, rubber covered roller and a metal backing roller. After coating, the sheets pass through conveyor belt in the ovens and are stoved.

Paints suitable for Roller Coating
They must have

-Excellent adhesion and flexibility.
-Excellent opacity
-Excellent flow and application characteristics
-Adequate chemical resistance for the end use.
-A hard marproof film formation quality.
-Adequate intercoat adhesion between various coats.

The main features of this process are high product rates, minimum losses, controlled film weight and thickness and economy.

Spray Application
Paint suitable for spraying
Normally paints are based on short oil alkyds and compositions based on fast evaporating solvents like Xylene and NC Lacquer.
Conventional Spraying

It operates on the principle of jet of fluid paint subjected to a stream of air. The correct balance between air and paint is essential for atomization and hence successful spraying. The system required for spray painting consists essentially of a source of compressed air, filter for removing dirt, water and oil, a container for the paint and a spray gun.

( Courtesy: )


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