Environmental conditions for paint application
Among the environmental conditions you must measure the dew point, air temperature, relative humidity, and surface temperature, to be sure conditions are suitable for blasting. Otherwise, condensation will form on the steel during or after blasting and cause flash rust, which can be detrimental to the overall quality of the coating performance.
Checking Conditions before Blasting
Most coatings do not adhere well to surfaces contaminated with oil and grease. Blasting actually drives them further into the steel rather than removing these contaminants and thus contributes to premature coating failure. Therefore, you should always inspect for visual surface contaminants before blasting. If oil and grease are present, they should be removed by solvent cleaning.
Checking Conditions before Paint Application
You should record environmental conditions before you begin work to determine if they are within the parameters established by the specification or the application data sheets for the coating material.
In most instances, this data will include air temperature, temperature of the substrate surface, humidity, and dew point. If there are substantial changes in the weather once the work begins, the time and nature of the changes should also be recorded.
It is unlikely under any circumstances that the Environmental conditions on a coatings project would not need to be recorded for reference purposes.
This applies for exterior work in inclement weather and for interior projects in the best of conditions.
An accurate record of this kind could prove invaluable for getting a project extension, if necessary, or for calculating overcoating times for specific types of materials.
Environmental conditions satisfactory for paint application.
Measure the following as close as practical to the work surfaces before work begins, during, and on completion of the work:
1) air temperature (wet and dry bulb readings);
2) steel (surface) temperature;
3) dew point temperature of the steel.
4) relative humidity;
Dew point is the temperature at which moisture condenses on a surface.
For example, if the dew point is (20 C), condensation will occur if the steel is at or below (20 C). As a general rule, paint application should take place only when the surface is at least (3 degrees C) above the dew point.
If the dew point is (20 C), the steel temperature should be at least (23 C). This rule provides a margin of error in case of instrument inaccuracies, quickly changing weather conditions, or human error.
Relative Humidity is the other important criteria for acceptability of painting. Some authorities or specifications will state that painting should not be allowed above 85% RH, because even small variations in substrate temperature can produce condensation at these humidities.
Note that for some coatings, being above a minimum relative humidity may also be an issue
Inspect surfaces for residual moisture.
Observe for atmospheric pollutants, chemicals, salt spray, fumes, dust or debris.
Define whether the measurements and observations taken allow paint operations to proceed or not.
Record all the information in the inspection report, including:
1 areas where work is being undertaken;
2 areas where Climatic conditions measured.
Extreme Conditions
Conditions will generally be unsuitable for painting:
(i) When it is wet.
Moisture will prevent adhesion of the paint to the substrate so painting is not permissible when it is raining, snowing, foggy or misty.
(ii) When there are high winds.
Overspray losses will be unacceptably high and there is a danger of paint contamination. There may also be a danger to the sprayer himself in windy conditions.
(iii) During extremes of temperature.
Cold Temperature: there is always a danger of ice forming on the substrate during sub-zero temperatures
Hot Temperatures: above 35°C (95°F) the drying and curing of paints is rapid and dry spray can be a problem.