To get stains out of marble, apply a poultice of one part salt, four parts flour, and just enough water to make a toothpastey mix. Let dry, scrape off, and rinse. To clean paint off marble, use regular paint solvent and scrape with a razor blade. Then sand the surface wet with 120-grit paper, either by hand or with a random-orbit sander (not a belt sander), and polish wet with 220-grit and 400-grit paper. Protect the clean surface with a penetrating sealer containing silicone. Don't use oil, which can darken marble.