Applying a finish after engraving slate coasters is generally recommended but not strictly necessary, depending on the intended use and desired aesthetic. Engraving creates a surface texture that can expose the porous nature of slate, making it more susceptible to moisture and stains. A protective finish, such as a sealant or varnish, can provide a barrier against spills and make the coasters easier to clean.

Additionally, a finish can enhance the contrast of the engraving, making the design stand out more prominently. If the coasters are intended for decorative purposes only, a finish may not be crucial. However, for coasters that will see regular use, applying a finish is a wise choice to prolong their durability and maintain their appearance over time.

Moreover, certain finishes can also add a sheen or matte look, depending on personal preference. In summary, while not absolutely necessary, applying a finish after engraving slate coasters is advisable for protection and to enhance the visual appeal, especially if the coasters will be used frequently.