When applying nail enamel, a base coat should be used:

Using a base coat prior to the application of nail polish is an essential step in the nail care process. The primary purpose of the base coat is to create a smooth surface for the polish, enhancing its adherence to the nail plate. This not only helps the nail polish apply more evenly but also extends its wear time, reducing chipping and peeling.

Additionally, a base coat serves as a protective barrier between the natural nail and the colored enamel, preventing staining or discoloration that may be caused by pigmented polishes. It can also help in nourishing the nails if it contains fortifying ingredients. Therefore, the practice of using a base coat before applying nail polish is critical for both cosmetic results and nail health.

The other options do not encapsulate the broad applicability and benefits of using a base coat effectively. The emphasis on using a base coat 'every time, regardless of the finish' suggests a more generalized approach that aligns with best practices in nail care.