John C. from San Francisco called about a recurring problem with Efflorescence on his exterior deck. We discovered that while some of the slate was covered, there was an open skylight where more rain would fall on the slate. This is where the heaviest concentration of white powder residue occurred.
Efflorescence occurs when moisture reacts with salts inherent in the mortar bed or mortar grout. As the crystals grow and expand, they leach out normally through grout joints. This is also the case with Brick, Terra Cotta tiles, or Saltillo clay tile.
We suggest neutralizing the acid cleaner with HMK R55 Intensive Cleaner to thoroughly prepare surface for sealing.
Finally, to add water and moisture protection, the best value for a Professional Grade stone sealer is HMK S31 Silane Impregnator.
I cautioned John, given the S.F. weather, once acid cleaning and neutralized with R55, allow to completely dry before applying sealer.
Another stone problem solved with quality stone care products from HMK Stone Care.