To perform satisfactory paint coatings must be applied on a correctly prepared surface.
Ideally each coat of paint should be applied to achieve the minimum dry film thickness requirement, with as rear as possible a uniform layer of paint. Paints have an adequate tolerance factor relative to films applied within a reasonable (f1) Excess (may be 50 %) of the specified thickness. However, to assume that paints applied at thicknesses considerably in excess of the specification will extend the protective life, is not necessarily correct. Defects which could result from excessive thickness include bubbling, blistering, cracking, cratering, extended drying, pin holing, sagging, and wrinkling, etc.
Although brush application is relatively slow process, this method fulfills a requirement on small and complex areas where spray application is not practicable or permissible. It is also the recommended method for the application of stripe coats. In most instances when high build coatings are applied by brush, multi-coats are necessary to achieve a thickness comparable with spray application. However, with sensible brush selection and an application technique which discourages over brushing, relatively thick film can be applied.
Roller application is faster than brush application and is more adaptable to relatively uniform surfaces such as tanks and walls. Brushes are required for the purpose of “cutting in.” Rollers of different length of nap are available to suit varying surface roughness. Short and medium nap are more adaptable to smooth surfaces. Long nap rollers are preferable on rough or irregular surfaces. As in brush application, high film build is not easily achieved with roller.
As paints vary in character and viscosity, it is essential that a suitable type of spray gun and the associated equipment be used in keeping with the product. Best results will be obtained by correct selection of fluid tips, needles and air caps, in conjunction with the necessary gun adjustments. For example, high viscosity paints are more suited to a pressure-feed system with a heavy duty gun combination. The majority of paint coatings can be satisfactorily applied through a pressure-feed system. Although some solvent addition is usually necessary for high viscosity paints.
The main disadvantages of air spray application when compared with airless spray are:
1. The extra solvent addition which can restrict the film build and retard the drying.
2. Over-spray and spray dust are unavoidable.
3. Slower rate of application.
Paint coatings are atomized by the action of forcing them at high pressure through an accurately designed small orifice. A combination of high velocity and rapid expansion of the coatings on passing through the orifice into the atmosphere cause them to break up (atomize) into a spray of fine particles without the additional assistance of compressed air as an atomizing influence. Hydraulic pressure in the range of 1,000 to 3,000 p.s.i. will cater for the requirements of most paint coatings. Higher pressures would only be required for unusually high viscosity coatings.
Airless application has several advantages over air spray application which includes:
1. Speed of application
2. Over-spray reduced to a minimum resulting in healthier, cleaner and safer working conditions
3. Ability to apply high viscosity paints in thicker and more uniform films
4. Improved “wetting” characteristics are provided especially when fast drying paints are being applied
5. Provided adequate pressure is available, more than one gun can be used with each spray pump
Only three factors of control relative to application are available to the operative which are:
(a) Hydraulic pressure
(b) Spray tip orifice and angle
(c) Viscosity of paints
The recommended practice in every instance is to ensure that the most suitable spray Up and pressure are selected and tried, to establish whether any solvent additions are necessary.
Orifice size selection is extremely important as combined with pressure they determine the flow rate of the paint. Serious consideration should also be given to the spray angle ensuring that the optimum fan pattern width is provided in relation to the shape and size of the structure involved.
The spray fan should be kept at right angles to the surface and the gun should be triggered off at the end of each pass. The ideal distance for holding an airless spray gun from the surfaces being coated is 12″ to 15″ this will ensure a full wet coat is applied. Distances in excess of 20″ encourage pin holing, dry spray, and over-spray. Ensure that the safety trigger-locking device is in working order and only released during spraying operation. When in use avoid placing hands or fingers in front of the gun as contact with the pressurized paint can cause serious injury. Always use the correct fluid hoses which are reinforced to withstand high pressures and which are free from kinks and damage.
Information contained in these pages is accurate and updated to the best of our knowledge.
However no guarantee of the result implied as application conditions are beyond our control.