The very basic question - What is curing?
Curing is a very basic process in civil engineering, in which water lost from concrete or masonry works due to hydration ( a chemical process that involves the expulsion of water ) is replaced and the surface is kept within the specified temperature range. This is such a vital and critical element for concrete or masonry to gain the expected strength. However, in my personal opinion, this is, maybe the most overlooked item in execution.
When should you start curing after plastering is done?
I would suggest that curing be started immediately after the application of plastering. I believe that it is a myth that a setting time is to be given for plastered surfaces. The hydration process starts as soon as cement and aggregates are mixed with water to form mortar. To prevent water loss due to hydration, it is advisable that curing starts immediately after the application of plaster to the surface.
How frequently should you cure?
Completely depends on the climatic conditions at the site. In hot and low humid climatic conditions, it is advisable that a hessian cloth or something similar is used to cover the plaster surface so that the dampness remains and water is not lost due to hydration. Theoretically speaking, water should always be present on the surface to aid strength gain.
In normal conditions, I suggest that curing be done at least thrice.
How many days should this curing be done?
Technically speaking, though it depends on the specifications of the cement used, almost 60-70% of the strength is achieved for mortar in the first 7 days. After 28 days, 95% strength is achieved. Hence, theoretically, curing may be done for 28 days. However, this is not practical. What is a good practice though, is to ensure that curing is done for 7 days properly.
What are the rules for curing between 1st coat and subsequent coats of plastering?
The best practice would be to apply the second coat immediately after the first coat and ensure curing for both together. My reasoning is that it is not advisable to have different thermal expansion coefficients for coats on the same surface. In other words, if you let the first coat of plaster to completely cure and then apply the second coat, the adhesion would be a concern and both layers would expand differently and this would result in cracks later on.
What is a good method to avoid cracks in plastering?
More about this needs to be written later on. However, what can be said in short is that more the heat of hydration for cement, more will be the water requirement and more will be the chances for developing cracks early on. What is ideal is that the cement has a higher fly ash component so that the heat of hydration is lower. I would recommend that PPC be used as cement instead of OPC. Also, a higher fly ash component be checked for so that the heat of hydration is lesser.