I have tried everything on this cast concrete facade. When I say everything I mean, Rust Remover plus, straight 12.5% SH, oxalyic and high pressure. I can’t make a dent in it. Any suggestions are greatly appreciated. I forgot to mention that there is a gutter splash guard just above this so it may be coming from the roof but it’s not rust or organic.
I’d try hydrofluoric acid
That almost looks like phosphates, which are basically tea stains, you could try a phosphate remover, I’ve only used that in pools though
You could also try tile and grout cleaner, or clr
Tricky tricky
Thanks for the ideas. I also tried an Efflo remover just because I had some. I’ll look into the phosphate remover, anything in particular you’ve had success with?
Look into one restore
Send pic to Craig Harrison and your comments on what you’ve tried so far.
but I’d start with F9 Barc mixed 1-1. Spray lite mist coat 1-3 times over 15min. Don’t let dry. Can use light agitation. Repeat if necessary.
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Have you tried rubbing real hard and fast? 
Is it localized or span most of the building?
Just that one spot on the cast concrete.
What ever happened to that stain?
In all honesty, I can’t remember. I probably ended up using One Restore on it but I’ve killed a lot of brain cells since then.
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…darn 
OneRestore works on a lot of stains and surfaces. Great stuff.
two years, too long.
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Heck, some days I can’t remember what I did 2 hours earlier 
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