There are four common methods of applying coatings to steel and other surfaces.
None of these are the 'best' method for all coatings for all scenarios.
Brush application is not an outdated method of paint application as is sometimes thought and indeed is still the basic method by which most decorative and maintenance coatings are applied. It is a relatively slow method but can be the most suitable for applying paint to small complex structures and also “hard-to-access” areas.
Roller application is generally used for large flat areas such as walls, steel sheeting, etc. It is a faster method of application than brush but not as fast as spray application. The universal use of this method is limited by two factors:
Roller application achieves a much better finish if the surface is smooth.
Spray application of coatings is the fastest method of applying a paint.
Conventional air spraying is commonly used for the application of coatings to cars, industrial products, and small areas of repair where a high quality of final appearance is essential. Essentially the paint is atomized to form a mist by the passage of a low pressure air stream and liquid paint through a small tip. The air then applies a pressure to force the liquid paint through a small tip. The air then applies a pressure to force the liquid paint through the orifice of the gun and propel it to the surface to which it is being applied.
This method of application is usually favoured for high build coating on large areas. It has considerable advantages over conventional spray, brush and roller methods of application in speed of work and the ability to achieve specified film thicknesses in one application. Hydraulic pressure alone is used to apply coatings by airless spray, in a controlled spray pattern.