Natural stones often contain traces of Iron, which may oxidise when they come into contact with moisture either from rain or cleaning. It can also occur when stone comes into contact with certain acid-based cleaners that contain ingredients that can accelerate the oxidisation process.
The result of this oxidation is that the stone develops rust and discolouration (brown staining).
LTP Ferrex is specifically formulated to help remove these blemishes.
For single spots of rust use LTP Rust Stain Remover.