Flamed salt & pepper granite presents an inherent cleaning problem when hydrochloric acid cleaners are used. One theory is that a chemical reaction to the hydrochloric acid can react to the ferrous compounds in this type of granite to form ferrous chloride. This formation can leave a yellowish tint (or stain) to the surface.
It is always advised to properly test any cleaning chemical prior to the full scale project. If the yellow stains appear after tested, it is recommended to try a different chemical blend, such as an alkaline or a phosphoric acid based cleaner that will not have the same reaction.
White sandstone also has inherent cleaning problems. Much like flamed salt & pepper granite, the use of hydrochloric acid based cleaners can leave an undesirable yellowish staining on the surface. We are not completely sure of what chemical reaction causes this; however testing confirms its validity.
The difference between the flamed salt & pepper granite and the white sandstone is that the stains on the sandstone may take longer to appear. In one case study, the initial stains appeared relatively quickly when a stronger hydrochloric acid based cleaner was used. When another hydrochloric acid based cleaner was used to remove the yellowish staining, the stains did not reappear for over four months.
Perhaps an even milder hydrochloric acid based cleaner could be appropriate, but it is suggested to try a different chemical blend, such as an alkaline or phosphoric based cleaner to originally clean the stone. Proper testing is essential to find the proper solution, since surface staining is far different than stains deep within the stone.
With most cleaning issues in the industry, we have solid case studies to indicate what will and what will not work. Even our experience cannot solve every problem the industry faces. There are certain situations that have no explanation and the only way to work through it is trial and error. We can offer solid starting points for testing, however we can offer no guarantee that those options will ultimately work.