out properties. Pickup of the undercoat is another problem which must be 

 considered as a possibility with rolling. 



Rolling is a faster method of application than brushing. Roller 

 coating has the same relatively low material loss as contrasted with 

 conventional spray. Particular care must be exercised when using a roller 

 on rough spots, pits, rivet heads, edges, corners, welds and the like, to 

 make certain they are properly covered. Roller coating however, like brush 

 application, is especially useful where spraying cannot be used because of 

 spray fog and possible flammability of solvent. 



(4) Spray . 



(a) Introduction. Protective coatings, including many rela- 

 tively small jobs, are applied by either conventional or airless spray in 

 almost all industrial work. Wastage of materials is higher because of the 

 fine atomization of the coating. The advantages of spray application are 

 its coverage speed and application uniformity. Newer spray methods include 

 hot airless spraying, electrostatic spraying, and combine speed with low 

 material loss. Hot airless, as the name implies, is simply a heated airless 

 spray application. Electrostatic techniques involve putting a charge on the 

 spray particulate and the opposite charge on the work piece being coated so 

 that the coating is electrostatically attracted to the part. Spray coating 

 has the obvious advantage too of relatively easy application in corners, 

 cracks and crevices. The newer spray methods also practically eliminate 

 overspray and spray fog. 



(b) Conventional Air Spraying . The spray gun in simple terms 

 is a tool using compressed air to atomize products and apply them to a 

 substrate. Air and coating enter the gun through separate passages which 

 brings the two together and mixes them at the air cap. Most spray guns 

 have two adjustments. One regulates the amount of fluid which passes 

 through the gun when the trigger is pulled. The other controls the amount 

 of air passing through the gun. This controls the fan spray width. The 

 internal mix gun mixes air and material inside the cap before expelling 

 them. This gun is generally used where only low air pressure is available 

 or where slow-drying products which do not contain abrasive particles are 

 to be applied. 



The external mix gun mixes and atomized material and air outside the 

 air cap. This type is more commonly preferred. Other types include 

 automatic, extension and special spray guns. 



(c) Airless Spray. This method of applying coatings uses no 

 air for atomization. Hydraulic pressure is used to atomize the fluid by 

 pumping it at high pressures through an accurately designed small orifice 

 in the spray nozzle which controls the fluid flow and spray pattern. The 

 air-operated hydraulic fluid pump multiplies many times the air input 

 pressure to deliver material at desired spraying pressures. Various size 

 caps are required since coatings do not atomize the same due to variations 

 in coating pigment grind, viscosity and other formulation features. 



Not all coating materials can be applied by airless spray. Many 

 heavy, pigmented, and fiber-filled abrasive materials cause some problems 



342 



