Cultured marble is a mixture of marble dust and binder that, while it looks like marble, isn't as resistant to damage or staining as actual marble. Because of this, you may eventually find that your cultured marble vanity has lost its good looks with age, attracting a collection of stains in the process. To fix this, you can change the color of the cultured marble vanity top as you would any other surface with a new coat of paint. While this will change the color, it also changes the look of the marble, resulting in a solid surface look for your vanity top. If you want to preserve the look of marble, seek a professional's help.
Turn off any water supplying faucets attached to the vanity from the water supply valves beneath the unit. Turn the valves counterclockwise to shut the water flow, and then turn on the faucet to empty out the remaining water in the pipes.
Remove any fixtures attached to the vanity top. Use a putty knife to cut through the caulking around the fixture base then pull the caulk free with the knife's edge. Loosen any screws or bolts holding the fixture in place with a screwdriver or adjustable wrench, and then pull the fixture free.
Clean the surface of the vanity with a cloth dampened with a cleaning solution consisting of mild liquid soap mixed with warm water. Rinse the soap clear of the vanity with clean water then pat the surface dry with a dry piece of cloth.
Prepare the surface for primer by sanding the cultured marble with medium-grit sandpaper. The sanding removes any finish already present on the marble as well as creates a textured surface that allows for efficient primer adherence. Wipe the marble after sanding with a damp cloth to remove the sanding dust, then pat with a dry cloth to remove any moisture.
Place a strip of masking tape along the edge of any adjacent surfaces to protect from the paint. Place a drop cloth on the floor beneath the vanity to protect the floor from spills as well.
Apply a layer of primer to the surface of the vanity using a paint roller to create an even coat. Allow the primer to dry overnight before continuing.
Paint a color topcoat over the primer using epoxy-based paint and a paint roller. Make certain the coating is even, using several light layers of paint to build up your final coat rather than a single heavy layer. Wait about an hour after each layer to start the curing process before applying the next layer of paint. Allow the paint to cure for 24 hours after you have applied the final layer.
Cover the paint layer with a polyurethane finish to seal the paint and create a wear layer that protects the paint from normal wear and tear. Use the roller to apply the polyurethane, covering the paint in three layers and allowing each layer to dry for two hours before applying the next. Allow the polyurethane to dry completely according to the drying time recommended by the manufacturer before reinstalling fixtures.
Use the screwdriver and wrench to reattach the fixtures to the vanity. Place a bead of caulk around the base of each fixture where it joins the vanity to create a waterproof seal. Smooth the bead with a wet fingertip. Allow the caulk to dry overnight then turn on the water supply and use the vanity as normal.