White stains on grey slate roof

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Author Message   Old_school (Old_school) Senior Member Username: Old_school Post Number: 1096 Registered: 01-2009 Posted on Monday, March 09, 2015 - 08:30 pm:        Welcome Victor! It certainly looks like whatever they did is not working...or it is working all right, but not the way they wanted.   Vcardenes (Vcardenes) New member Username: Vcardenes Post Number: 1 Registered: 03-2015 Posted on Monday, March 09, 2015 - 09:52 am:        Hello My name Is Victor Cardenes, I've just registered in the forum. I'm geologist and work in roofing slate research. For this cover, it seems that the successive chemical cleanings affected the carbonates of the slate, and probably the iron sulfides. These groups of minerals are common, and under some conditions may lead to weathering and damaging of the slate. According with my point of view, applying aggressive products on slates might make things worse. As a general rule, cleaning of slate should be done very carefully, and always with mechanical methods. Hope this information is useful to anyone! Victor Cardenes roofingslate.wordpress.com   John_chan (John_chan) Senior Member Username: John_chan Post Number: 148 Registered: 08-2006 Posted on Friday, July 04, 2014 - 01:22 pm:        This looks very similar to a Brazilian black slate roof I inspected in Florida a few years ago. Any ideas on the origin? Joe is right. There's still a good bit of rust on the slate. Phosphoric acid is a good cleaner for slate flooring. I've never seen it used on a roof, but the acid must be diluted. Full strength phosphoric acid can permanently damage the slate's appearance. I don't know much about how the acid affects the slate's service life.   Joe (Joe) Moderator Username: Joe Post Number: 828 Registered: 07-2006 Posted on Thursday, July 03, 2014 - 11:36 am:        Looks like the rust is still there. Is that Spanish slate?   Tmorgan (Tmorgan) New member Username: Tmorgan Post Number: 1 Registered: 07-2014 Posted on Tuesday, July 01, 2014 - 11:22 am:        We are a cleaning/coating contractor in Florida. We cleaned the Mold off of a grey slate roof with Sodium hypochlorite and roof snot a surfactant. It cleaned up well and looked great at the end of the day, However there were rust spots under it. The contractor we were working for asked us if we could get that off. We tried F9 which is a rust remover( primarily phosphoric acid). It worked on the rust, and looked good but the next day the roof had white hazing covering it. We have tried to remove this hazing and are having no success so far. Does anyone know what this is, and have you found a solution to addressing this? Thank you.