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.
Another option that we do in a scenario like this is:
If the buildup is extremely heavy, three dimensional – in that you can feel it with the pad of your finger or even scrape off a little bit with your fingernail, we would begin by wetting the surface with water and then taking a fresh, new, single edge razor blade and carefully scraping the surface. In doing this it is important to keep in mind two things; 1) Keep the razor blade at a very low angle, like you are shaving hard cheese or chocolate. 2) Be sure that the razor blade does not become damaged; i.e. bent, chipped, dull, etc. If so, keep changing to a new blade.
If you don't do these things correctly it could be possible to scratch the surface of the stone. If you did scratch surface it would take diamond resurfacing to remove the damage.
Now that the heaviest buildup is removed we would turn our attention to the residual film. For this we would use, an industrial type variable speed grinder (the speed will need go all the way down to zero), an 8 inch backer pad with a fiber based pad and an acid based marble polishing powder. The powder we use is a combination of tin oxide, aluminum oxide and oxalic acid as well as a few other ingredients. Generally speaking a marble polishing powder would not be used on granite because there is no calcium present in the stone. In this scenario we are counting on both the oxalic acid as well as the mild oxide abrasives to remove the mineral buildup. Apply a small amount of powder to the stone being very careful not to inhale any of the dust. To this add enough water to work it into a wet paste and begin working it with your variable speed grinder set at the lowest speed possible. Use a great deal of care to prevent your paste/slurry from splattering and damaging the surrounding areas. Continue to work the area until you think you have removed the remaining film, this could take several minutes. Remove the flurry, rinse the area and buff dry that section – preferably with a micro fiber cloth.
Once you are done and the surface is free of moisture you should apply a very good quality penetrating granite sealer. Do a final buff and you should have years of trouble free service out of your granite.
Posted by David Gelinas 1800MARBLEGUY.COM
Great comment David. Thanks for your contribution.
Thanks David. Great post! I agree. Would like to add that blades are excellent on granite and will be difficult to scratch, as you know, steel is equil to granite on a mohs hardness scale, but better to be on the safe side… I also like marble polishing compounds on granite to clean them up, crystalizers work well also (for other solutions)
Another thought I want to add, is the nanotechnology glass sealers. I have used them on granite (good for silica based materials) and they really do work well, would be much better for keeping the mineral crust from re-bonding, and what does bond will come off much easier, I have learned this from experience.