The choice of application can be affected by the type of finishes to be used. A finish formulated for compressed air spraying is suitable for compressed air hot-spraying or airless spraying by adjustment with thinner. Some types of finishes may be ill-suited 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.
At present, hand rolling is done mainly for decorative and maintenance paintings. Roller Coating application is most effective on broad plain surfaces. The covering material for the roller can be plastic sponge. When equipped with a long handle extension, you can use the roller for floor painting and places that are otherwise inaccessible.
For larger areas it is more convenient to work from a bucket than a tray. A perforated grid is placed inside the bucket, the roller is dipped into the paint and then rolled over the grid to remove surplus material and distribute evenly. In the tray, a reservoir at one end holds the paint. After charging the roller, it is rolled out on the platform of the tray. To use, roll is roller 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 hence, is likely to be popular with amateur painters. Roller application on narrow or broken surfaces does not show any special advantage over brushing.
Roller coating application is used exclusively for coating metal containers such as tins, drums, kegs and barrels.
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 belts in the ovens and are stewed.
Paints suitable for roller coating must have
The main features of this process are high product rates, minimum losses, controlled film weight and thickness and economy.
Brushing is the oldest, and a versatile method of application under unlimited variable conditions and situations. It is still the best method to paint large complex objects. Brushing helps the paint penetrate and reduce wastage. It is mainly used for decorative and maintenance application.
Choice of brush
Best quality brushes are made up of hog bristles. These bristles taper from the roof towards the tip where each is split into two, for finer strands known as flag, which deliver slick finish.
A proportion of animal hair or vegetable fibres are used in cheaper variants. Brushes made of nylon bristles are also available. These are superior than pure bristles in their resistance to wear, but are smooth and non-absorbent. Hence, they carry less paint and have a tendency to overspill from the brush.
Always select a brush of suitable size, for the work at hand. 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
Bristles in new brushes hold dust and must be washed out before using for finish work. Soap water can be used for washing out the new brushes. A paint brush works best when it is 'broken in' i.e. the tip of the bristles are levelled. A brush must be 'washed in' in order to distribute the paint throughout the thickness of the brush. The brush is dipped in paint and rubbed against the sides of the kettle. After painting is over, brush back the excess paint into the container and scrap clean it with a blunt putty knife. Rub it on a suitable surface and rinse out in a thinner and 'spin' dry. Wash it thoroughly with soap water and leave it for drying.
Paints suitable for brushing
Air drying type, based on long/medium oil length alkyds, emulsion 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.
Paint suitable for spraying
Paints are normally based on short oil alkyds and compositions based on fast evaporating solvents like Xylene and NC Lacquer.
This method works 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.