What is the purpose of a sealer in painting?

The purpose of a sealer in painting primarily focuses on preparing surfaces by preventing issues such as bleeding from knots and inhibiting staining in porous materials, particularly wood. When wood is painted, the natural resins found in knots can sometimes seep through the paint, creating discoloration and uneven finishes. A sealer creates a barrier that effectively stops these substances from affecting the paint layer applied over it, leading to a more professional and lasting finish.

While other functions like providing a decorative finish, acting as a primer, or adding color to the surface are essential aspects of the painting process, they do not capture the core function of a sealer. Sealers are specifically formulated to prepare surfaces and manage the particular challenges that arise, making option B the most accurate representation of a sealer's purpose in painting.