Curing a finish means to bring it to its final intended degree of hardness and luster. Heat is the primary method of speeding up the curing process. Today’s finishes can also be water-based, and heat, although helpful in accelerating the drying of waterborne finishes, is not as effective as air movement.
Curing by Heat
Heat curing is accomplished either by heated or superheated air, or by radiant heat emitted from infrared lights. Both methods are used in both industrial and refinish applications to cure finishes. Powder coat finishes are cured through a heat cycle, as well as painted finishes.
In most cases, the temperature required for curing is referring to the temperature of the substrate being painted. For example, in an automotive refinish situation, the baking temperature specification refers to the vehicle’s metal temperature, rather than the air temperature being used.
For example, if a paint has a recommended bake temperature of 140 degrees Fahrenheit, the air temperature may have to be set at 160 degrees Fahrenheit to achieve the recommended 140-degree Fahrenheit metal temperature.
The “cool down” phase is the period of time required to cool down the heated vehicle and the paint booth. This phase starts automatically upon completion of the bake period and generally takes three to five minutes.
When cured by heat, additional time may have to be allowed for the finish to reach its full hardness. The finish on cars fresh out of a cure cycle is still soft enough to be dented with a fingernail or scratched by buffers. The need for post-cure cut-and-buff can be dramatically reduced through diligent shop cleanliness and contamination control.
Waterborne Paint Drying
Downdraft airflow is generally accepted as the best type of airflow for a paint booth. It does an excellent job of controlling overspray and contamination, providing a safe, clean environment in which to paint. However, the limitation of downdraft airflow is the creation of laminar air and boundary air. This severely hinders the waterborne paint drying process. Laminar air is created as air passes in one direction over a solid object in a paint booth. Boundary air is a low-pressure layer of slow-moving air immediately below the laminar air and above the surface of the vehicle.
When examined under a microscope, even the most perfect paint jobs are not entirely smooth. They have tiny bumps, dips and ridges that are not noticeable to the naked eye. These tiny imperfections slow down the air enough to create a layer of slow-moving air referred to as the boundary air. During the paint drying process, the boundary air becomes saturated with water molecules from the paint, limiting the speed of evaporation that can take place. It is this boundary air that prevents the airflow from drawing water molecules out of the wet paint.
To achieve the fastest drying times possible, the boundary air must be disrupted and dispersed. This disruption is accomplished by creating turbulent airflow on the surface of the vehicle. Accelerated air systems need to be added to break up the air. It breaks up the slow-moving boundary air, rapidly speeding up the drying process.