Grout and stone or clay tiles look impenetrable, but in fact neither is completely waterproof in their natural, unsealed states. Applying liquid sealant over the grout lines is simple, unless you’re installing unsealed tiles. In that case it gets complicated because, if you grout the lines first, it might stain the unsealed tiles; but if you seal the tiles first, it can prevent the grout from setting correctly in the lines. The proper approach is a balancing act of sealing and grouting in multiple layers.
Use your paintbrush to apply a layer of penetrating sealant to your newly installed, unsealed tiles before the grout is installed. Brush the sealant on the tile face in a thin, even coat, but don't let it gather or drip in the spaces between the tiles.