Tile Setting Materials | Grouts

Joint width decides it. Under an eighth of an inch, use unsanded. Wider than that, use sanded, because the sand keeps the grout from shrinking and cracking. On polished marble many installers use unsanded anyway, since sand can scratch the face.

Yes, for marble, travertine, limestone and any porous stone. An unsealed stone absorbs pigment from the grout and the haze never fully comes out. Seal, grout, then seal again once it has cured.

About once a year in a kitchen or shower, less in a low traffic room. If water stops beading on the joints and starts darkening them, it is time.

A penetrating sealer soaks into the stone and slows absorption from the inside. A surface protector sits on top and takes the wear. Most floors want the penetrating type; countertops sometimes get both.