Stone walls can be painted, although it is not the normal procedure. By cleaning the surface of the rock to eliminate any grease, you open the rock to absorb sealers and then a layer of paint. Painting should be the method you want to use before you start since the paint will be almost impossible to remove later.
Wet the surface of the rock wall with trisodium phosphate to soften any grease, dust or general dirt off as well as etch the rock for better absorption of the sealer. Dilute ¼ cup of the cleaner with 1 gallon of warm water and cover the surface with a sponge dipped in the solution. Scrub the rock wall with a medium-bristle brush after it has soaked for a couple of minutes, getting into all the crevices.
Rinse the wall with warm water, starting at the top with a clean sponge, rinsing out the sponge constantly. Let the wall air-dry.
Apply a coat of masonry primer, painting it on with a 2-inch paintbrush, starting at the top and working down. Dry the primed surface according to the directions on the primer container.
Paint the cleaned and primed wall with a couple of coats of latex paint, again using a 2-inch paintbrush, starting at the top and working down. Dry the paint completely between the coats, according to the manufacturer's instructions.
Wear rubber gloves and goggles when cleaning with trisodium phosphate.