Some types of paint need to be mixed with a hardener, or catalyst. These products are also known as 2K (for two components). The catalyst causes a chemical reaction which starts the drying process. These paints do not air dry like most paints. Without the catalyst, they will never fully dry. Once the catalyst is added, the paint will dry within a few hours, even if you put the lid back on the can. Some types of paint will air dry, but can also take a hardener to increase the paint strength. Once you place an order with MyPerfectColor, you will receive an email with technical instructions on how to use the paint. This will include information about the need for a hardener. Any paints that MyPerfectColor sells that require a hardener/catalyst will come as a kit and include the hardener with the paint. Our acrylic enamel does not require or have a hardener. It is a single component system.