Wash the marble with a mixture of warm water and a drop of liquid dish soap applied with a soft sponge. This removes dirt and grime without damaging the marble. Avoid using chemical solvents or wire brushes that can damage the marble surface.
Apply a sealer to the marble with a paintbrush or roller to protect the surface against water infiltration while the plumbing is installed. The sealer fills the pores in the marble to prevent moisture from getting into it. The sealer also protects the marble from stains during the rough plumbing process. Once the first coat dries, apply a second coat for best results.
Cover the marble with a tarp, while working on the plumbing, to further prevent stains or damage.