Traditionally, a stucco building requires three layers of stucco. The first layer is the scratch coat. The brown coat layer is next. The final top layer is the stucco coat which can be colored with pigments. The stucco coat is wholly dependent upon the brown coat for proper drying. Just because the surface of new stucco is firm to the touch does not mean it is dry. The water in the stucco mix must release slowly for proper drying. If all the elements work together as they should, the average drying time for the stucco is 90 days.
How Humidity Affects Drying Time
The moisture level in both the stucco mix applied to exterior walls and in the atmosphere is important. Within the stucco mix, water is the engine that drives the chemical reactions within the Portland cement mix which causes the stucco to harden properly. If the atmosphere is dry, the water in the stucco mix will be pulled from all three layers too quickly and the stucco will not be strong. To slow the rate of evaporation, you can put plastic sheeting over the stucco walls or the stucco can be misted with water a few times a day. However, if colored pigment has been added to the stucco mix, misting is not used because it may cause the color to be uneven and mottled. When the days are cool and damp, the conditions are good for drying stucco; dry, windy summer days are not good for drying stucco.
How Temperature Affects Drying Time
The ideal time to apply exterior stucco is in the summer and early autumn. The outside temperature ranges for proper drying of stucco from a low of 40 degrees F to a high of 90 degrees F (4 to 32 degrees Celsius). Outside temperature should be about the same both when the brown coat is applied and when the stucco coat is applied because this impacts the moisture content in both coats which then impacts the drying.
How the Brown Coat Affects the Drying of the Stucco Color Coat
The brown coat and the stucco coat are in an important partnership with each other. For instance, each coat must be mixed with the same amount of water, mixed for the same length of time, each batch must be used with the same time span. These two layers of stucco must be as similar as possible so that the rate of drying for the stucco coat mirrors as closely as possible the drying time of the brown coat.