We have slate on the floor under and around a portable fireplace. Last winter we had some flooding in that room, and it seems that the water caused white stains to appear on the slate. We tried Lime-Away to no avail. Do you have any answers how we can remove or cover the white stains?
When white stains appear on stone or masonry surfaces it is usually efflorescence (salt and lime stains). The stain is from salt and lime in the masonry materials (grout, block, or concrete), and the stains come to the surface with water movement.
You could try a stronger cleaner. Scrub the area with a strong solution of MEX and hot water. MEX is a detergent that is also used for cleaning masonry. If the stain remains, test a small area with UGL ETCH. This is an acid cleaner for efflorescence. You need to test an area because it may also discolor the slate. Finally, you could go to a store that deals in tile and slate and find a proprietary cleaner for slate or stains.