The correct performance of a coating, in addition to a good surface pre-treatment, of an adequate application process. Identify the variables to consider at this stage.
by Abel de la Cruz*
Even today, it is common to confuse the specification of the paint application procedure with the specification of the paint application method. And it is that, specifying the painting procedure, implies a factor of greater amplitude, where we must consider in addition to selecting the method or application equipment itself; the paint preparation procedure, the application time intervals between layers, the degree of dilution or thinning of the paint, define the safety measures in the application, etc.
The procedure of proper application of paint is a critical part of the technical specifications of painting a work. High-performance systems are especially more sensitive and can fail more drastically than conventional ones.
The factors that must be taken into consideration in the procedure for the preparation and application of paint are explicitly stated in the SSPC-PA-1 Standard, under the title: Paint Aplication Specification N° 1 Shop, Field, and Maintenance Painting. However, the Technical Product Information Sheet is also a very important tool for developing application instructions and characteristics, which is designed and provided by the paint manufacturer.
Preparation of the painting
In general, the paints before their application must be completely homogenized and must not present separation of phases, remains of lumps, sediments, etc.
The pigments in some paints tend to settle at the bottom of the container, some sediments are soft and can be incorporated with manual stirring and others are very hard and will require mechanical agitators.
Not all the paints that are going to be applied, are of a single component. Currently, the most used are those of two components, with different mixing ratios between the resin and the hardener, where the final mixture of the product must be uniform.
The correct preparation of the paint before carrying out the painting work itself, includes the following steps:
1. Ensure that the material received for the work corresponds to that required. This includes:
to. Sufficient amount of paint: Base and finish.
b. The corresponding components.
c. The thinner or reducer required in quantity and quality.
2. Read the instructions for use on the label or in the Technical Information Sheets of the product.
3. Mix the contents of each container and component with a flat pallet, until perfect homogenization is achieved.
4. Mix the components (resin, catalyst) in the case of epoxy, polyurethane or additive paints.
5. Allow induction time in case of catalyzable paints, as referred to in the manufacturer's technical information.
6. Reduce the paint, if necessary, use the diluent in the proportion recommended in the technical information of the product. It should be noted that the use of diluent in paint may change the application characteristics of paint in terms of flow, leveling and appearance.
In the case of products with two or more components, dilution shall be carried out after the catalysed product has been suitably homogenized. The temperature of the medium will influence the viscosity of the paint. In cold climates a higher proportion of diluent is required than in hot climates.
7. Filter the paint if necessary, using the recommended mesh size.
8. In case of using other containers to prepare the paint, it is recommended that these are completely clean without dry paint adhered to the walls of the container.
Methods of Application
In order to select the appropriate application method: with brush, roller, conventional gun or airless gun, it is necessary to know the characteristics of the paint, configuration of the metal surface, environmental conditions, urgency of the work, trained personnel, etc.
In adverse environmental conditions (rain, winds, high humidity, etc.) and with little surface preparation, it may be preferable to apply with a brush since a better contact between the paint and the metal will be achieved.
The diagram in Figure No. 1 represents the merits of making the application with a brush or gun. Note that the duration of the protective coating is independent of the application procedure, if the operation is carried out under suitable environmental conditions. On the other hand, in adverse weather or the surface preparation of steel is poor, such as when it is cleaned manually after an appreciable time of exposure to the weather, the advantages are on the side of the brush application, since the alternative movements of the brush expel the small globules of water or impurities eventually present on the steel and ensure the intimate contact between paint and metal.
The methods that may be used for the application of paints are:
Brush application
Commonly used brushes are made of nylon bristles and their measurements range from 2 to 6 inches wide. For quality work it is important to choose the right brush according to the type of paint to be applied, the area and shape of the surface, etc.
Brushing is slower than other methods and should be used mainly for small areas, for cuts in corners and edges. Brushing is also used to improve the wetting and penetration of bases or primers on surfaces that present adhesion difficulties.
Types of Brushes
- Wall brushes - They are flat with square edges with a width of 3" to 6". Used for painting large areas and are nylon.
- Brushes for stripes and retouching.- There are in 4 forms:
- Flat with square edge
- Flat with angled edge
- Flat with cylindrical and oval edge.
These brushes have a width of 11/2" to 3" and diameters of 1/2" to 2".
Roller application: Rollers are recommended for large flat areas and do not require the dexterity of gun application. The work is two to four times faster than with a brush. The recommended ones are of the type of polyester or nylon fiber and their measurements vary in diameter, length and size of fiber:
Diameter: Can be 11/2" or 21/4" in.
Length: They can vary between 1" and 18" in, the most common being 9".
Fiber length: Range from 1/8" to 3/4" in.
The choice of roller as in the previous case will depend on the type of paint to be applied.
Air spray application: Also called gun application, it is the most commonly used method and atomizes paint mixed with compressed air. A compressor supplies air under pressure through a hose to the spray gun that atomizes the paint to produce a fine spray, which is evenly projected to the surface.
This method is recommended when you want to cover larger areas. It should be borne in mind that the application with a gun causes losses of the order of 20 to 40% due to the formation of the "over spray" (on dry spray, paint particles that fall on the painted surface).
The air compressor must supply dry and clean air, in sufficient volume and pressure for the "spraying" equipment to operate well. Basically, for each operation with the "spray" gun, 15 to 20 CFM of compressed air is required and to operate each agitator 10 to 15 CFM of compressed air. The recommended air pressure in the compressor is 90 to 100 psi.
Airless spray application: Spraying is produced by hydraulic pressure. By means of a pump at very high pressures, the flow of paint inside the hose is forced through a small hole resulting in the atomization of the paint without using the air. This method of application results in greater productivity, covering larger areas in less time and producing less "over spray" (so
bre dry spraying, paint particles falling on the painted surface). It allows greater penetration of the paint on surfaces with pitting, cracks, areas with resane, corners, etc. As well as the ease in the construction of high and uniform thicknesses of paint film.
Multicomponent Spraying Application (Plural Component Spraying): It is the method of spray application of paints of various components, which are pumped, dosed and mixed accurately through pumps by pneumatic action. They are high performance equipment, easy to use and capable of spraying coatings with 100% solids, short "pot life" and in difficult working conditions.
General considerations
No application work will be started or interrupted:
(a) Where the ambient temperature is below 5°C, with the exception of paints dried by evaporation of solvent or specified by the characteristics of the paint. The paint will not be applied, if it is expected that the ambient T° will fall from 0°C before the paint has completely dried.
(b) Paint shall not be applied to a steel surface whose temperature is 3°C below the dew point.
c) Nor shall paint be applied on steel, at a temperature above 60°C, unless it is a paint specifically designed for it.
d) When it rains, snow, there are winds with sand and dust projection or when the relative humidity is outside the recommended parameters (maximum or minimum). It will also not be applied on wet or wet surfaces, or on surfaces with ice sheet.
e) In case it is required to apply in wet or cold weather, it will have to be painted covering or adequately protecting the surface to be painted or the ambient air will be heated to the acceptable temperature. Ensuring drying. Areas damaged by the presence of moisture or condensation must be repaired by removing the paint, preparing the surface again and repainting with the same number of layers.
f) In all cases, it is recommended that the successive layers differ in color or hue, clearly distinguishable to the eye.
* Manager AmericanConsult Peru
[email protected]
http://www.americanconsultperu.com
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