What Does Dry-to-Touch Time Mean?
Dry-to-touch time is the length of time measured from a coating's application to the drying stage where the coating has hardened enough so that it can be “touched lightly without any adhering to the finger.”
Coating hardness depends on the coating material. For lacquer and sealer coatings, dry-to-touch is when there is no “pronounced marks left by a finger touching the film.”
These definitions are from ASTM D-1640, “Standard Test Methods for Drying, Curing, or Film Formation of Organic Coatings at Room Temperature,” which is the most widely used and recognized method for determining the drying stages. The dry-to-touch stage is also called the “dry” stage, between the “tack-free” and “dry-hard” stages within the eight drying stages defined in this testing method.
ASTM D-1640 is subjective and there are other test methods that coating material users can reference.
The dry-to-touch time, usually found on the container's label or product data sheet, is just a guide as there are many factors that determine the coatings’ actual drying time.
Corrosionpedia Explains Dry-to-Touch Time
The dry-to-touch time for five or more substrate temperatures are usually found on the coating materials’ technical data sheet. Although they are based on exposure at industry standard temperature, relative humidity and coating thickness, there are other variables that determine the actual dry-to-touch time, such as the following:
- Relative humidity and temperature in the work area. Generally, higher humidity or lower temperatures will lengthen the drying time, while lower humidity or higher temperatures will speed up drying. The industry standards for testing drying times are 77°F (25°C) and 50% relative humidity, which could differ from the actual conditions.
- Temperature of the substrate to be coated. The substrate temperature can affect drying time in the same manner as the ambient temperature. Substrate temperature at or near the dew point could add moisture from the environment and increase the drying time.
- Test methods used to determine the dry-to-touch stage. There are various test methods in the coating industry, from the finger pressure method in ASTM D-1640 to the use of a tack tester and mechanical recorders.
- Coating thickness. The dry-to-touch time will be longer for thick coatings than for thin coatings.
- Ventilation (airflow). Good ventilation means more fresh air coming in to replace the air that has been saturated with the coatings’ volatile components. The fresh air will absorb more volatiles and speed up drying of the applied coating material.
Estimated drying times are important in planning coating jobs.