While porcelain tiles are nonabsorbent and fairly stain resistant, the grout which encircles the tiles isn’t. Grout is a porous, cement based product which may absorb water, and therefore stains, even if not sealed from them. After grout has dried and cured for at least 72 hours after installation, it may be sealed using a clear, impregnating sealer which will help impede staining and give you more time to clean up spills. Seal your grout no matter how old it is to help prevent new stains from forming.
Sweep the floor to get rid of any dust or debris from the grout. If the grout is old, clean it well with a grout cleaner and scrub brush to remove any surface dirt which could be sealed into the grout. Let the grout dry completely.
Carefully paint the sealer onto the grout using a paintbrush. Be careful to only saturate the grout using the sealer, not the porcelain tiles. If some sealer does get on the porcelain, wipe it away immediately with a soft fabric.
Let the sealer penetrate the grout for about 10 minutes. If this is the first time you have sealed the grout, apply another coat of sealer at the moment. When the second coat has penetrated, wipe away the excess grout using a lint-free fabric. Continue buffing the grout along with the tiles until they feel dry and not tacky to the touch. The sealer will heal within 24 hours; avoid washing the tiles till that time has passed.