Any ideas? I've been asked to look at cleaning a number of walls with a red staining (see pics), the surface is rendered (K-Rend I'm told) and looks like the stain is bleeding through from below the surface. Any ideas : A) what it might be ? (salts/rust/pollution/weird algae??? B) suggested approach to clean off? I know K-rend is tricky to clean but any of you guys seen or treated this before? I've not visited the site, as it's a distance away but can get any questions answered which might help? Would like to do the job so any help appreciated!!