There are various methods and equipments employed for application of surface coatings depending upon the nature of coating, the surface, availability of curing methods, and the product the performance expected of the paint system. In the last couple of decades new and improved application equipments have been deiced, which have the added advantage of giving better finish and better protection of the substrate coupled with saving of paint material.
Is one of the oldest methods of paint application and still the most versatile and indispensable for many jobs. Brushing gives better penetration of paint than by spraying and hence recommended for all primer applications. It is also cheaper, minimum skill is required to operate (as compared to mechanical methods) with minimum loss of paint. Brushing is more of an art than a science. Mostly employed in decorative and architectural applications Selection of brushes is important depending upon the surface. Like flat brushes for plain surfaces, round or oval brushes for irregular surfaces, corners, rivets, bolt etc. Bristles of the brushes should be clean; spreading should be on the surface at short strokes to be leveled with longer strokes at right angles to the previous strokes.
Spraying is done by atomization of paint through a narrow nozzle by air pressure. Paint at a pressure (of the order of 10-15 pounds per square inch) is passed through the nozzle, outside of which air at higher pressure (of the order of 40 to 70 p.s.i.) come and meet the paint stream, thus atomizing it into a fine spray. It is important to control flow of the paint. Correct proportion of the atomizing air and the proper spray width to achieve satisfactory results. Viscosity of application in stoving paints should be between 20 to 30 seconds and in case of air-drying paint, 40 to 50 seconds. Gun should held at right angle to the object and at about 9" to 12" distance from it to attain uniform spray. There are gravity feed cup guns and pressurized pots available. Loss of paint is maximum in conventional air assisted spray depending upon the air pressure and size and shape of the articles, higher the pressure and smaller the articles, more losses and vice versa. Modernization of conventional spraying techniques has resulted in automatic spraying processes.
The equipment means of producing high pressure on paint and a special gun connected in a pressure resisting hose. The unit works on the principle of applying a high hydraulic pressure to the paint to force it through a tiny orifice. The high pressure is obtained from a normal paint spraying compressor via an air powered pump which develops a final paint pressure of from 20 to 30 times that of the incoming compressed air. The final pressure is of the order of 400 to 4000 p.s.i. depending upon the consistency of the paint and the area to be painted. This has consistency of the paint and the area to be painted has to be adjusted to the required optimum levels. This method is ideal for large surfaces like ships hulls, bus bodies and such areas.
This works on the principle of electrical conductivity of the article to be painted and the paint and like charges repel and opposite charges attract. In other words, if small particles of paint are charged negatively, they will be attracted to and deposited over positively charged object. The paint should be suitably adjusted to have high electrical conductivity (or in other words low receptivity} to be electrostatically spray able and the article is to be earthed