Slate tiles that have been used in a shower, may absorb minerals or soaps that will produce white spots on their surface. Slate tiles used on floors can develop similar spots from salt tracked in from the outdoors, improper cleansers or spills from various liquids.
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Apply a stone cleaner to the slate tile. Stone cleansers are available as liquids; pour some of the cleanser directly onto the white spots on the slate and then scrub the surface of the stone with a scrub brush.
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Rinse the slate with clear water and allow it to dry completely.
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Apply a poultice to the slate wherever white spots still remain. The poultice will penetrate the stone, lifting out stains to the surface where they can be rinsed away. Cover the poultice with plastic wrap or a plastic drop cloth and allow the poultice to work for one hour.
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Rinse the poultice off the stone with water and reapply stone cleaner. Scrub the surface of the stone a second time with the scrub brush to remove any substances the poultice has brought to the surface.
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Rinse the stone a final time and allow it to dry.
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Seal the slate tiles with an impregnating sealer to help protect them from further white spot stains. Paint the sealer onto the tiles with a foam paint brush, allow it penetrate for an hour and then wipe up the excess with a lint-free cloth.