mold on outdoor limestone blocks

Hello, we have an outdoor bench made of limestone blocks and have noticed black coloration (and some red spots) forming in one area...which we assume is mold.   The limestone is fairly new (under 2 months since it was installed) and was sealed with a generic limestone sealer purchased from Home Depot.  I thought the sealer would prevent mold from growing...

2 questions:
1) Is MB-9 the right product to clean the mold?  I've seen other questions in the knowledge base that also seem to address this, but wanted to be sure...
2) What's the best sealer to use to prevent mold from growing in the future?

Thanks!

 

Dear Nikki:

 

“ I thought the sealer would prevent mold from growing... ”

 

Nope! As a matter of fact it is quite possible that the presence of the totally useless sealer actually caused the mold and reddish staining by slowing down the natural osmosis of the stone. That is why no outdoor stone installation should ever be sealed.

Yes, MB-9 is the right product to remove the mildew staining, but it may turn out inefficient with the reddish staining that could be the oxidation of some iron mineral within the stone.  

 

“ What's the best sealer to use to prevent mold from growing in the future? ”

 

None. Actually it would be best if you could get rid of the one that you applied originally, which is virtually impossible to do. The more the stone can “breathe” freely, the better the chances are that mold will not develop. What has to be said, however, is that limestone in general is not a good stone for outdoors. Only a few types, i.e.: Indiana limestone could apply for the job.

 

May I ask you now to please read and e-sign our Statement of Purpose at: http://www.marblecleaning.org/purpose.htm? J

Ciao and good luck,

Mauri z io Bertoli

 

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