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The accepted methods of applying the protective coatings described in this manual are by brush, roller, conventional spray and airless spray. The advantages and disadvantages of each method are briefly described below.

Brush Application
Brush Application is a relatively slow procedure, but is generally used for decorative paints and for coating small areas. It is particularly suitable for coating complex small areas where the use of spray method leads to considerable losses due to overspray. However, most high build coating is designed for application by airless spray, and high film built will generally will not achieved by brush application.

Roller application
This is a faster method than brush application on large, even surfaces and can be used for application for thixotropic protective coatings but control of film thickness and high film build is not achieved.

Conventional Spray
This is a widely accepted and rapid method of paint application in which the paint is atomised by a low-pressure air stream. Conventional spray is relatively simple and inexpensive, but it is essential to use the correct combination of air volume, air pressure and fluid flow to give good atomisation and a paint film free from defects. If conventional spray application is not controlled correctly, large losses of paint can result from overspray and rebound from the surface. The major disadvantage of conventional spray is that high build coatings can generally not be applied by this method, as most paints have to be thinned to a suitable viscosity for satisfactory atomisation and so lose their high build properties.

Pressure pot
These are generally used in association with conventional spray guns to provide a means of delivering paint from a tank to a spray gun through a hose. The compressed air, which has entered the tank forces, paints from it to the gun through a hose and atomises he paint. Paint in the tank is prevented from settling by means of a stirrer driven by a compressed air motor or by hand Pressure pots are used in cases where large quantities of paint are to be applied and their use, instead of a gravity feed cup attached to spray gun, significantly reduces waste time in constant refilling and also enables the gun to be turned to any angle without spilling of paints.

Airless Spray
Unlike conventional spray and pressure pot air is not mixed air is not mixed with the paint to form a spray, hence the name airless spray. Atomisation is achieved by forcing the paint through specially designed nozzles or tips, by hydraulic pressure. The required hydraulic pressure is usually generated by an air-powered pump having a high ratio of fluid pressure to air input pressure. Pumps with ratios between 20:1 and 60:1 are available, the most common being around 45:1.

The chief advantages of airless spray are :

The tips through which the paint is forced to achieve atomisation are precisely constructed from tungsten carbide . The atomised "fan" of paint is produced by a slot ground on the face of the orifice. Various orifice sizes together with different slot angles are available . The choice of tip is governed by the fluid pressure required to give atomisation coupled with the orifice size needed to give the corect fluid delivery rate. The fluid delivery rate controls the film thickness applied. Different slot angles produce spray fans of different widths , and the selection of a particular fan width depends on the shape and size of the structure to be painted. Choice of fan width is also related to orifice size - for the same orifice size , the paint applied per unit area will be less, the wider the spray fan.

Generally, tips with an orifice size of 0.009" are suitable for coatings to be applied at approximately 50 microns (2mils) wet film thickness. Tip sizes from 0.013" - 0.019" for wet film thickness of 100-200 microns(4-8 mils) and 0.019" - 0.031" for above 200 microns (8 mils) are generally used. Heavy duty mastics which are applied at very high film thicknesses may need tips with orifices as large as 0.040" to 0.060".

There are several designs of tips available, the choice of which depends upon the finish required, the ease of application and ease of cleaning blockages from tips.

With some products, the decorative effect achieved with airless-spray is not as good as can be achieved by conventional spray. However, airless spray application is now widely accepted as a convenient method of applying high build coatings.

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