Masons and do-it-yourself type homeowners install tile outdoors to cover patios, walkways and driveways. Grout lies between the tiles to fill in spaces and give the surface a uniform finished appearance. Sealing grout prevents the grout from staining. Cement-based grout requires sealing in outdoor spaces because the grout is somewhat porous. Sealers prevent mould penetration and growth. Stains, mould and mildew can only lie on top of the sealant.
Sweep the surface to remove all dirt, dust and debris from the grout lines.
Wash the grout surface with a mild detergent and scrub brush. Rinse the soap off the grout and allow the grout to dry completely. Remove areas of mould and mildew with a commercial mould killer. You can use an equal mixture of chlorine bleach and water to remove mould from white grout. Chlorine bleach may discolour or lighten coloured grout.
Apply an exterior-grade penetrating grout sealer with a small brush. Some sealers come with an attached brush application on top of the bottle.
Allow the grout sealer to dry in accordance with the manufacturer's instructions, generally one to two hours. Apply a second coat of grout sealer and allow to dry.
Test the sealer by pouring water on the grout. If the water beads up on the surface, the sealing is complete. If the water spreads, dry the area and apply another coat of grout sealer.
Clean and reseal grout every one to two years to maintain a protective coating. Apply penetrating sealer to unglazed tiles as well as grout to protect them from stains and mould growth.
Grout sealers may darken the grout colour.