by, Mike Marsoun
Removing a mineral crust from a polished granite surface can be difficult and dangerous. Some black granites (anorthosite) can actually have calcite minerals and if acid cleaning they can etch, which is not something you would normally expect with granite.
If the mineral coating has a grayish look to then it is likely silica. The only acid that will remove silica is hydroflouric, and this will burn the polished finish of the granite.
Of course, many granites are not sensitive to common acids (hydrochloric, phosphoric, sulfuric), and if the mineral staining is calcium based, acids will work fine, but there is still the danger of grout deterioration.
The method I have used on occasion works very well and does not involve the use of any chemicals at all. This is an abrasive method, using 5″ aluminum oxide velcro backed wet/dry sand paper, and a handheld sander/polisher.
It works like this: you start with a 220 grit and remove the majority of the minerals, leaving a thin film on the surface, being careful not to break through to the granite surface. If it is very thick you may have to start with a 120 pad. After the majority of this layer is removed you will move on to a 400 grit pad and lightly go over the surface until the pad is sufficiently “loaded” with the sanding dust from the minerals you are removing. Once this pad is “loaded” there will be no concern of the polished granite getting scratched or dull, the abrasives will only be aggressive enough to remove the mineral layer, and will also serve to polish the granite!
When polishing is completed be sure to clean and buff with a microfiber cloth, and seal with a high quality impregnator.
This is a great trick and once you have practiced it a while and understand how these paper pads work, you can use this technique on marble. It works fantastic in removing mineral crust from shower walls, prior to final polishing.