Which medium is often an alternative to masking fluid?

Rubber cement is often considered an alternative to masking fluid because it serves a similar purpose in protecting areas of a surface from being painted or colored. When applied, rubber cement forms a temporary barrier that can be removed after the artwork is complete, allowing for clean, unmarked areas of the surface underneath. This is particularly useful for artists working with various paint media, such as watercolors or acrylics, as it facilitates the preservation of specific details and highlights in the artwork.

The other options do not function as suitable substitutes for masking fluid. Varnish is typically used as a protective finish rather than a medium for masking. Linseed oil is primarily used in oil painting to alter the texture and drying time of the paint, but it does not provide the same protective qualities as rubber cement or masking fluid. Turpentine is a solvent used for thinning oil paints and cleaning brushes, and does not serve the purpose of masking in the same way.