• Grout is the material that sits between your tiles, spanning the gaps to help form a flat surface. Grout is more absorbent than tile, tends to get dirty faster and is more difficult to clean. If the grout is dirty, the whole tiled area looks dirty. Very dirty grout can also lead to cracks and gaps in the tile, which can allow moisture to get through and affect the tiles.
  
  

Surface Cleaning

• Sometimes all you need to do is clean the surface of the grout if it has mold, mildew and other contaminants on the surface. Even if you need to deep clean the tile, you should surface clean it first to get rid of the contaminants. Use a stiff brush to scrub the grout lines with a 50-50 solution of bleach and water, which will kill mold and mildew, one of the most common problems on the surface of the grout. Rinse well, and follow up with a degreasing dishwashing liquid to pick up any remaining dirt.
  
  

Deep Cleaning

• For deep-set stain and discoloration, surface cleaning is not always enough. Surface clean the grout first, and make a paste of powdered oxygen bleach and water. Spread the paste over the grout lines thickly with a rubber spatula. Let the paste sit for 20 to 30 minutes while it bubbles and lifts the stains from the grout. If it dries, apply more. Rinse thoroughly, and look for remaining stains. If there are any, repeat the process until they're gone.
  
  

Sealing

• Always seal the grout after you clean it even if it has been sealed before. Sealing is the process of covering the grout with a clear, protective glaze that keeps off future stains. Apply it with a paintbrush or sponge applicator, running it along the grout lines while keeping it off the tiles. Use at least two layers of grout sealer. Ensure that the grout is fully clean and dry before you start; otherwise you will seal in the stains or moisture.