How to clean dry grout from flagstone used on a patio surface?

QUESTION

Removing Grout Haze - I have a question about cleaning dry grout from the surface of flagstone used on a patio surface. The place that sold me the flagstone suggested using a laundry detergent and a wire brush and scrubbing. I could be out there for days doing that as the patio is approximately 20 x 30 feet. Is there another way to get the excess grout and haze off of the surface? It has dried but I am hoping that you may have a solution. How about muratic acid???

ANSWER

ANSWER - The easiest way to remove a cement film off flagstone or tile is using an acid if done correctly.  If there is thick excess grout on the flagstone then that should be scraped off first.

Muriatic acid is very dangerous and can end up burning the grout stain into the stone that becomes permanent if you are not careful.  The muriactic acid has to be substantially diluted if it is used.  Sulphamic acid is less corrosive and recommended for this type of cleaning but will take more effort to clean it.  If the haze is light enough you can use cheese cloth to remove the haze or if more difficult then use diluted vinegar with cheese cloth and then buff dry.

Make sure you don’t seal the flagstone before you get it completely clean or you may seal in the grout haze.

When using acid it is important to saturate the surface of the stone or tile with water so the acid doesn't tend to penetrate into the stone.  After you apply the acid and scrub with a nylon type of bristle brush then it is very important that you flush the surface of the stone with enough water to remove any acid residuals.

After cleaning the stone and letting it dry thoroughly then apply a penetrating sealer either an enhancing type to bring out the colors or the standard type that will not change the look of the stone.    Good Luck!

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