black stains on Sandstone Mike Marsoun 03 Apr 2010 07:56 EDT
This is strange. For a friend who is a supplier. There is 3 photo's that say
grouted section.  This black started not long after grouting, mainly around
were they cleaned off the grout.  It seemed like the grout contaminated the
stone.   So 80% of the wall they did not grout and
was looking good until a couple of weeks ago, and now that is starting to go
Mike Marsoun
Nu Life Stone Care