Dear Susie:
Although you do not specify that, I assume that your walls are outdoors, right?
And maybe you live in an area with humid climate.
If that's the case, then a good cleaning with a mildew stain remover rated for natural stone like MB-9 will do the trick for you.
However, you have to make sure that the mildew comes from without, because the problem could be structural. What I mean by that is that if you have infiltration of water behind those veneer limestone tiles, the mildew could be developing from within. Not only would it be much more difficult to clean off (it would probably need to be poulticed out, not just cleaned off), but it would not solve the structural problem, either. If you can detect the possibility of infiltration of water behind the limestone, address the problem immediately, before doing any cleaning.
May I ask you now to please read and e-sign our Statement of Purpose at:
http://www.marblecleaning.org/purpose.htm?
Ciao and good luck,
Mauri
Article ID: 1532
Created On: Sat, Jul 5, 2008 at 12:37 PM
Last Updated On: Sat, Jul 5, 2008 at 12:00 AM
Authored by: Maurizio Bertoli [mail@mbstone.com]
Online URL: https://marblecleaning.org/knowledgebase/article.php?id=1532