Temperature

Low ambient and/or low surface temperature may retard the drying or curing process of paints, causing solvent entrapment and extending the recoating intervals.  This situation is not common in Mauritius.

High ambient and/or high surface temperature may speed up the drying or curing process of paints by rapid solvent evaporation.  This may cause condensation in the wet paint film which may result in surface defects such as gloss patches and haziness.  Rapid solvent evaporation can also cause brush, roller and overlap marks.  The use of a retarder-thinner or additional thinning down of the paint will assist in improving application properties.

Ideal conditions would be between 20° and 28° Celsius.

Condensation

Relative humidity is the measure of water vapour held in the atmosphere.  The level of humidity is very much dependent upon atmospheric pressure and temperature.  In general, the warmer the air, the more water vapour it will hold.  If the substrate happens to be cooler than the surrounding air, the air will be cooled causing it to deposit its water vapour, in the form of droplets by condensation.  This is known as dew point, i.e. the temperature at which water will be spontaneously deposited from the air.

The presence of water on the substrate surface, even in very small amounts which may not be noticeable will threaten the ultimate performance of the paint system, causing primarily poor adhesion and poor drying properties which may develop into secondary film defects.  Painting should be carried out only when the substrate is at least 3°C warmer than the dew point.  At relative humidities exceeding 83% there will often be a difference of less than 3°C from the dew point and hence painting should be postponed.  This problem is quite common in Mauritius and is often encountered whilst painting roofs, especially during early morning/late evening when air temperature could be increasing/decreasing faster than substrate temperature.

Rain

Similarly to avoid the presence of moisture on the surface of the substrate, painting should not be attempted during rainy weather.