Tile floors, whether ceramic or porcelain, are a great choice for bathrooms because they hold up so well to moisture. But the grout lines between the tiles can be a real problem if they aren’t sealed to prevent dirt and grime from seeping into the porous surface of the grout.
Before you can seal it, you have to clean it thoroughly. Try spraying the grout lines with hydrogen peroxide. Wait about 30 minutes, then scrub with a stiff nylon brush. A combination of baking soda and white vinegar is another good cleaning solution.
Once the grout is scrubbed and rinsed clean, wait at least 48 hours to make sure the floor is dry. And then use a quality sealer to form an impenetrable layer over the grout to keep dirt and grime away.