CURE TIMES FOR STUCCO
Stucco is a cement-based product and as such requires curing, the same as any Portland cement-based product. There are requirements for cure times and there are recommendations for best results. The main intent of cure times is to allow the cement basecoat to set and shrink before proceeding with final decoration (finish coat).
The cure time between the scratch and brown coat in a Portland cement plaster is stated in the Uniform Building Code (UBC) and International Building Code (IBC) in chapter 25 tables as 48 hours. However, there is a section (2512.8 IBC) (2508.6 UBC) that allows the "alternate double-back method". This states that brown coat can be applied as soon as the scratch coat has attained sufficient rigidity to receive the second coat. Many experts believethis is beneficial to the plaster by providing a stronger bond between the two coats and enhanced curing of the scratch coat.
In either case, the brown coat must cure a minimum of 7 days prior to the application of the finish coat per the UBC and IBC. The NWCB "recommends' a minimum of 10 days. The NWCB believes if it is possible to wait 14 to 21 it is even better. This gives the cement a little more time for final shrinkage and a chance for the building to "find itself, it also has been noticed that most "reentrant" cracking in the brown coat appears around the 12 to 14 day period. Reentrant cracks are the hairline cracks that can appear at comers of square openings in the stucco such as Windows. These cracks typically run diagonally from the apex of the comer of the opening.
If vapor permeable finish coat is used, the cement basecoat will cure regardless of how soon the finish is applied. However, there is an increase in the likelihood of shrinkage cracks appearing in greater number by applying the finish coat to soon. Another consideration is the Ph level of the cement basecoats. Portland cement has a relatively high Ph level and can be reactive to many paint products cause. The Ph level will automatically drop to acceptable level in about 7 days after application.