Paints are formulated with the objective of applying a certain coating on a surface, thereby providing a protective and decorative film. Success in the application of any paint will surely depend on several factors, such as surface preparation, thickness of applied film, application method or conditions during application.
Conventional methods commonly used in the majority of cases are roller, flat brush and normal brush applications, conventional spraying and airless spraying.
This application method is frequently used in painting average size surfaces. For this application, lamb wool rolls should be used, because paints contain organic compounds that attack foam rolls.
Usually, high thickness paint does not spread well enough when applied with a roller, leaving an uneven film, which may cause lack of coating at low thickness points. This is because high thickness paints are formulated to prevent them from running when they are applied and have high thixotropic viscosity.
When thermoplastic systems (such as chlorinated rubber, vinyl, acrylic, etc) are being repainted, great care should be taken, because the strain applied on the previous coating may cause the paint to become soluble again and start forming clots during application. It may also cause bleeding.
This is a relatively slow application method usually adopted for smaller areas, retouching, application of additional layers on weld areas, rivets, edges and corners.
High thickness paints were developed to be applied by airless spraying, therefore it is difficult to obtain a uniform layer without grooves resulting from brush hair.
When thermoplastic systems (such as chlorinated rubber, vinyl, acrylic, etc) are being repainted great care should be taken, because the strain applied on the previous coating may cause the paint to become soluble again and start forming clots during application.
This application system is the most effective and productive one, due to the way in which the paint is designed and laid on the substrate, providing good film uniformity and spreading evenly over the applied surface. Data on the type of equipment needed for application, considering the viscosity factor, type of material and so on, can be found in the tables section.
This is a fast and very popular method, due to ease of application. The conventional gun is a relatively simple set of equipment, but skilled labor is indispensable to employ the combination of air volume and pressure with fluid flow rate that will produce a flawless film. In addition to the above controls, the choice of gun and its equipment, such as: air cover, needle and type of nozzle--which have a direct influence on the spray quality--is very important.
If application by spraying is not adequately controlled, an overspray of paint can happen, not to mention technical problems, such as running, poor spreading, porosity, etc.
High thickness paint, if applied by conventional spraying, will require more dilution, which means a greater number of coatings to attain the recommended thickness.
In spraying without air (airless spraying), no air is mixed with the paint to form the fan, as it is sprayed by hydraulic pressure, through specially designed nozzles. This pressure is supplied by an air-driven pump, with a high fluid pressure to air pressure ratio.
The main advantages of airless spraying are that high thickness paints can be applied with no dilution, for large-scale paint jobs on plates or flat parts; less material waste and reduced overspray; fast application and savings.
The spray fan is produced by a slit at the tip of the nozzles. The choice of the nozzle will depend on the fluid pressure needed to obtain the desired spraying and on the orifice needed to produce the correct fluid flow rate.
The choice of the size of the application fan is related to the size of the orifice and choice of the nozzle, and will depend on the type of desired finish and ease of application.
The main cost factors in an economic analysis of the different types of painting systems are: surface preparation, paint purchasing, application and duration of the painting system.
To have a better idea on the ideal value of any paint job as regard to its performance under given work conditions, it is necessary to have some experience in calculation so that the system becomes economical and to take into consideration the price per square meter, dry film thickness, paint application and other expenses.
The useful life of each system depends on how efficient the preparation of the surface to be painted is, since the better the preparation, the better is the paint´s adhesion to the surface. The two other factors that play a major role in making the system more efficient are: a greater thickness of the dried film and use of high-performance paints.
The formula for applying paint with potential savings is described as follows:
MATERIAL COST / SURFACE PREPARATION COST / APPLICATION COST |
} |
= TOTAL COST / YEAR |
SYSTEM´S DURATION |
||
SURFACE PREPARATION |
Once the total cost of any given painting system is determined in terms of sq.m. / year, you can determine the potential savings achieved from the most efficient painting system and then, you can compare the systems and analyze what your specific technical requirements are.
High performance painting systems are apparently more expensive. However, in environments subject to harsh conditions that require long-term protection, the use of this type of material is fundamental.
Yield is the surface area in square meters that a given volume of paint will cover, although it is difficult to actually estimate the amount of paint required for a given job. The theoretical yield is calculated by taking into account the nonvolatile material (solids) in a specific volume of paint.
In this case, it is assumed that the paint will be applied to a totally even surface without any roughness and there will be no paint loss during application. However, as we all know, surfaces are normally uneven; they have roughness due to abrasive blasting and/or porosity, which causes paint accumulation and/or absorption on or by the surface, leading to a decrease in theoretical yield.
Below we have highlighted some of the factors that contribute to paint loss during application:
a. The first factor is loss due to poor paint distribution on a surface. This results from the application of excessive paint by painters who desire a certain thickness and in the process make each layer or film uneven.
b. The second factor that reduces the theoretical yield is the manner in which the paint is applied. If the paint is applied using a paintbrush, a flat brush or roller, there is normally a 5% to 15% loss, whereas with conventional guns there is roughly a 30% loss.
c. The third factor that interferes with yield is the paint environment conditions. In external paint jobs, when the weather is very windy or hot, loss can be double the figure mentioned in the preceding item, and most critical in the case of spray guns.
d. The fourth factor that reduces yield is the loss that results from packaging and/or equipment used in applying the paint such as:
- drums: as some of the paint is left in the drum when the paint is transferred to the pressure tank;
- two-component paints: when their useful life expires;
- hoses and pressure tanks: due to the material that sticks to the walls, which cannot be used.
e. The fifth factor that affects yield is the type of substrate and the conditions of the surface on which the paint is applied. If the paint is being applied to pipes, metal bars, beams, etc., the loss will obviously be high and may even be as much as 50% higher than the loss registered on even and uniform surfaces. The combination of all these factors gives us the real yield, that is, the area (in square meters) covered by one paint drum.
Conclusion: the real yield must be determined from practical experience on a work site if the figure is to be truly reliable.
Induction time is the time in which bi or tri components are added to catalyzed paints, after the mixing of components A, B or C, so that the application process can be started. This time is at least 15 and at most 30 minutes at a temperature of ± 25ºC. The lower the temperature, the longer is the induction time.
The induction time is of fundamental importance in that it avoids any loss of physical and chemical properties after the paint is applied and, consequently, ensures curing so that the durability of the painting system is not jeopardized.
It is also important in that it is the time needed for the reagents to react and, as a result, attain the desired maximum performance.
Note: It is important to mention that in most cases dilution should only occur after induction time.
The paint should be applied with relative humidity below 85% and temperatures between 10ºC and 40ºC. Epoxy coatings usually dry through chemical reactions at temperatures above 12ºC. When temperature is below 12ºC, the paint does not cure. There are special epoxy coatings that cure with temperature above 5ºC, except when there is a special note on the product technical data sheet.
You should only apply paint if the substrate is at least 3ºC above the dew point.
Paints are tested in labs under satisfactory environment conditions and room temperature is ± 25ºC. Any temperature below or above this may retard or speed up drying and total curing.
If the maximum repainting interval is overstepped, the film should be sanded to ensure perfect adhesion of the subsequent coating.
For internal repainting of tanks, constant exhaust must be ensured until the products are completely dry. The ideal temperature for complete curing of this system (7 days) is 25ºC. Therefore, for every 3ºC below this temperature, curing will require an extra day.
Paints pigmented with zinc chromate or zinc dust can form zinc salts on any film surface exposed to weathering if finishing is delayed. In this case, you can wash the surface with drinking water, brush off with a nylon brush and dry.
Chlorinated rubber paint, when applied using a conventional gun, is likely to become stringy (normal in this type of paint). To minimize the problem, we suggest you increase the number of coatings, dilute the paint by 20% and thin out each layer.
Finishing paints with little body and light tones do not cover the surface very well when they are applied to a darker background. In this case, at least two coats are necessary to cover the surface.
The recommended dilution is directly contingent on atmospheric temperature, the equipment used, as well as on the painter´s skills. As the weather varies greatly in the different regions of Brazil, we recommend that you always consult our technical department to find out how much diluting solvent is needed for every type of equipment used.