The Quality Control of the painting process includes the evaluation and prior verification of the best weather conditions or favorable conditions to start the application of the protective paints.
This technical inspection must be carried out by the contractor responsible for the structural manufacturing, and verified by the external audit company responsible for the quality assurance of the painting, on behalf of the owners or managers of the infrastructure in maintenance or in construction project.
In the design of a Quality Inspection Plan, the following should be considered:
• Monitoring of environmental conditions at each stage of the process.
• Pre-inspection of manufacturing defects.
• Operability and control of surface preparation equipment and materials.
• Inspection of the quality of surface preparation: degree of cleanliness, roughness, residual contaminants.
• Inspect and evaluate the paint and its preparation: sampling and application tests.
• Operability of the application equipment.
• Inspection of the application of each of the layers of paint.
• Final inspection (Finishing).
• Technical reports and documentation: Registration of data in inspection formats.
* Inspection of the materials to be used.
* Inspection of surface preparation and application equipment.
* Inspection of surface preparation conditions.
* Inspection of application conditions by paint layer.
* Final inspection of the work or the total painting system
• Acts of reception of work.
The monitoring of environmental conditions is the starting point of the technological process of applying paints, so it is necessary to know how these factors affect the surface preparation, the application, during drying, curing and the behavior or durability of the coating.
Environmental conditions are basically referred to climatic conditions, such as temperature, humidity, rain and wind, since at the time of applying the coatings have a considerable effect on the protective value and life of the paints applied. Hence, it is recommended to suspend the exterior painting without any cover, when some type of inclement weather occurs or is imminent, such as rain, snow, frost, fog, excessive wind, etc. or when the weather conditions are such that there is and can be condensation of moisture on the metal. The latter, due to low temperatures and / or high relative humidity, and contaminants will be trapped in or between the layers of paint, impairing their good behavior over time.
Remember also that high protection coatings are more sensitive to any type of surface contamination than those of the alkyd type to oil, which can tolerate a certain measure of small amounts of surface contaminants. So, depending on the type of paint, the effect of environmental conditions will be different.
The starting point to foresee the effect of weather conditions on the paint will be to have at our disposal the Technical Information Sheet of each of the paints involved in the project, since in it, the manufacturer establishes the ranges or limits of temperature or humidity required by the application of their paints. Such is the case of some paints of the Coal Tar Epoxy type, whose application conditions are usually specified as follows:
Conditions of Application
(Epoxy Coal Tar Technical Sheet)
• Surface temperature:
Minimum: 5°C
Maximum: 50 °C
The surface temperature must be at least 3 °C above the dew point. (In order to avoid condensation of moisture on the metal surface at the time of painting).
• Relative Humidity of the environment:
Minimum: 10%
Maximum: 85 %
• Paint temperature,
Minimum: 5°C
Maximum: 35°C
The influence of climate conditions on these application parameters is evident.
However, weather conditions, especially temperature and humidity, will not only affect the application operation itself, but will also have an influence on:
• The induction time of the mixture (Resin + catalyst) for Epoxies.
• The pot life or shelf life of the paint mixture (Resin + catalyst).
• Minimum and maximum drying times for the application of the next coat of paint (Drying intervals or refinish times between layers).
• Drying time to the touch.
• Drying time for handling the painted structure.
• Total curing time to put the service of the painted structure.
• Storage conditions during storage.
Hence, the aforementioned times are specified in the Technical Sheet of the painting on the basis of a certain value of temperature and relative humidity, being able to be as an example, in terms of temperature: 21 °C (70 °F) or 25 °C (77 °F) or in terms of relative humidity 50%, depending on each manufacturer.
In practice, high humidity and/or low temperature will prolong the drying time, otherwise it could decrease it. Therefore, it will be necessary to inform about the variations of the aforementioned drying times against different temperatures or humidity. This information is usually published by the manufacturers in the Technical Sheets of the product, in case this information is not available, it is advisable before starting operations, to request it from the manufacturer's technical department. This is because this is the only way to guarantee an adequate application and above all, that you have correctly followed the instructions and application conditions that the paint establishes in your Technical Sheet. This will also provide you with the support against any claim that could be presented to the manufacturer in case of any premature failure due to the application. Continuing with the example of the Epoxy Coal tar paint, let's see how the application parameters of this paint vary depending on the Dry Bulb Temperature or Ambient Temperature:
Drying intervals (Drying time to repaint) between layers:
Maximum Minimum Temperature
At 16°C 24 hours 48 hours
At 5°C 12 hours 24 hours
At 32°C 6 hours 12 hours
Drying Times:
16°C 25°C 32°C
For Handling: 72 hours 48 hours 24 hours
Final curing: 15 days 10 days 7 days
If these variations or influence of the ambient temperature on the drying times are not taken into account, errors will be incurred for the good practice of applying the paint. For example: If the ambient working temperature is 32 °C and a second layer of Epoxy Coaltar paint is applied after 48 hours of drying after applying the previous coat of paint, instead of having been applied between 6 and 12 hours at most, it is very likely that there is a very low adhesive force between layers. This due to the difficulty of wetting and anchoring the second layer on the first, due to the excess in the drying time of the latter, consequently the probability of premature failure translates into the "peeling" of the second layer of Coal Tar Epoxy.
Measurement of environmental conditions
The task of the Quality Inspector of the contractor company and according to the Paint Inspection Plan pre-established for the work, is to measure the following parameters:
• Air Temperature or so-called Dry Bulb
• Relative humidity
• Dew Point
• Surface Temperature
Then, immediately check if the values obtained meet the established application conditions. It is necessary to remember that in case of a coating failure or the presence of defects during or after application, the first thing that is reviewed are the paint inspector's records on environmental conditions. These records are kept in the daily Inspection report and measurements must be made before starting the work and during the day every four hours.
The commonly used instrument is the Psychrometer, by means of which the temperatures of wet bulb, dry bulb and based on these two values can be measured, and with the use of the Psychrometric Tables of the air-water system will allow us to calculate the relative humidity and the dew point. There are also other instruments such as the so-called Hygrometers which allow us to directly determine the relative humidity and the dew point. The method covering these determinations is found in ASTM 337-84: "Standard Test Method for Measuring Humidity with a Psycrometer (The Measurrement of Wet and Dry- Bulb Temperatures)".
The temperature of the metal surface can be determined with a disc thermometer or clock with the magnetized base containing a bimetallic sensor; the available models depend on the temperature ranges to be measured. One of the most common is the PTC Surface Temperature Thermometers, with scales in degrees Celsius (°C) or degrees Fahrenheit (°F).
However, and for greater speed and accuracy of data, field inspectors can now count on a single digital instrument, such as the Elcometer 319 Dewpoint Meter – Dewmeter in accordance with iso 8502-4, which allows them to measure: Air temperature, relative humidity, surface temperature, dew temperature and also the difference (D T) between the dew point and the surface temperature.
Remember, the correct measurement of environmental conditions are the starting point to ensure the good performance of a protective coating.
*General Manager - Principal Consultant, AmericanConsult Peru [email protected]
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