Acids do remove cement-based grouts very well. However, they may also activate salts (calcium) in the slate or other similar type stones causing a chalky look when dry. I have seen this many times. A good solvent-based impregnating sealer (preferably a color enhancer) should rectify this issue and then all should be good.

Always test first, to confirm results and procedure, before general application.


Bob Murrell

Director of Operations

SCC/M3 Technologies Inc.





M3 Logo maroon grey cutout 2 in


From: [] On Behalf Of Barry Raduta
Sent: Thursday, May 31, 2018 2:42 PM
Subject: Outdoor black slate on patios


Hello Partners,

Thank you in advance for any help you can give me to help this client.

A few weeks ago I asked for help to clean up grout stains on flagstone and Dr Fred said to use a mild acid and Stu recommended Klenzstone #4 or #5.  I did not get to that job yet, but I saw about 2000sf of mostly black slate yesterday and I am wondering if it should be tackled in a similar manner.  Some of the tile spaces are clean and others show a deposit (calcium?).  I was thinking of a medium, nylon brush as my tool.  I was going to do a test area with just water to see how clean I can get the slate without the deposits and other test areas with a recommended product to remove the deposits.  Any thoughts?




Thank You,
Barry V Raduta
Diamond Stone & Tile Care-South

Stone & Tile Pros


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