Before grouting, non-vitreous tile should be:

Dampening non-vitreous tile before grouting is essential for several reasons. Non-vitreous tiles, which are more porous than vitreous tiles, can absorb moisture from the grout. If these tiles are dry, they may draw water from the grout mixture, leading to issues such as incomplete hydration of the cement in the grout. This can weaken the grout bond and result in cracks or uneven surfaces once the grout cures.

By dampening the tiles, you help to prevent excessive absorption of moisture from the grout, allowing for better adhesion and a stronger finished product. This method ensures that the grout remains workable and adheres properly to both the tile and the substrate beneath.

In contrast, the other options do not address the unique properties of non-vitreous tiles effectively. Leaving the tiles dry can cause adhesion problems, soaking them in water could lead to over-saturation and other complications, and sealing with a primer is typically unnecessary for non-vitreous tiles and may interfere with proper grout adhesion. Thus, dampening prior to grouting is the best practice to ensure a durable and aesthetic installation.