Water marks are a common occurrence in households with hard water. As splashed water evaporates from the slate's surface, it leaves minerals behind. These mineral deposits adhere to the surface of the slate leaving a white saltlike spot behind. Luckily, these spots are surface stains. And although they are bonded to the surface of the slate, they are relatively easy to remove with the right tools and a bit of elbow grease. To prevent water spots from recurring, wipe up excess water instead of allowing it to evaporate. Or apply a slate wax to the stone.
Scrub the area with a stiff brush and a mild liquid dishwashing soap and distilled water solution.
Rinse the area with a sponge soaked in distilled water.
Buff the area dry with a towel.
Buff the water marks away with steel wool. Stop once the water marks disappear.
Wipe the area with tack cloth to remove any slate dust.