For tough stains, try a strong, targeted formula like RMR. “You can also try a DIY solution,” says Martin, who suggests a 1:5 formula of water and white vinegar or soap. If that (combined with a little elbow grease) doesn't displace the stain, try ammonia with baking soda, she says. But be sparing with the baking soda—too much can scratch the surface of porcelain.