The zinc-silicate coating is dry to the touch within three to six minutes of application. De Boer Staal supplies primed steel dry to the touch to its customers.
Once the material has reached the dry to the touch stage, the coating will continue to dry and harden. This is called the curing process. This is a chemical process in which temperature and humidity play a critical role.
Moisture in the air reacts with the silicate thickener forming a polymer network, releasing a small amount of methanol due to evaporation. This polymer network gives the material the mechanical strength and chemical resistance (to solvents, water etc.) in the dry coat. The time that the primer needs to cure depends on ambient conditions.
De Boer Staal uses the data in the table below from coating supplier PPG to assess curing time.
In order to determine the speed of curing and the acceptance boundary, the customer can have an MEK rub test performed that is in accordance with ASTM D4752. The extent of curing is represented on a scale from 0 - 5 in this test. 0 means fully cured, 5 means not at all cured, while 3 represents a level of curing that is acceptable in practice.