Here is a classic example how unprotected stone, and lack of regular maintenance can create a very unsightly film on Slate and other Natural Stone surfaces, particularly in Wet Areas.
Moisture, can and does, penetrate all Unprotected Natural Stone. When this moisture reacts with naturally occurring salts in the stone itself, the mortar setting bed, or grout, Efflorescence grows. Yes, I said, GROWS. Salt crystals will grow and expand and will find a way out towards the surface. Sometimes this film leaches out directly through the pores of the stone, but more often it finds its way through the path of least resistance, the joints.
Fortunately, this can be rather easily resolved and prevented.
Using a professional grade Acid specifically formulated for Efflorescence, the white film can be removed. *Please Note: Acids are for use on UNPOLISHED stone, as acids will burn or etch polished calcium based stone like Marble, Travertine, Onyx, and some Limestone.
Using this acid Full Strength or diluted 50% will dissolve the salt crystals and all the other mineral elements that were carried to the surface in the process of efflorescence.
If grout needs to be repaired, this it the time to do it. Allow grout to cure a minimum of 48 hours.
Next, you need to Neutralize the acid with an Alkaline cleaner. I suggest using HMK R55 Intensive Cleaner. This neutralizing wash will also clean any residual oils, soap scum and other elements that may interfere with re-sealing the stone.
Lastly, to help prevent this process from reoccurring, it is best to protect the stone with a quality Penetrating Sealer or Impregnator. I suggest using HMK S34 Silicone Impregnator. This is a Professional Grade Impregnator that will penetrate deep into the pores of the stone and grout to retard the growth of efflorescence and help prevent water infiltration into the stone.
Following these steps will return the stone to near new condition, and protect your investment.