Rust removal from granite

["Scott Engering" <Scott@xxxxxxxxxxxxxxxxxxxxx>]

Syd

Without placing you in danger through an adverse mixture of reagents, dare I
suggest another chemical to try, at least if it is a granite or sandstone
boulder that you are working on!

Try orthophosphoric acid. Certainly available through UK suppliers of
chemicals for the cleaning of building facades, it is used mainly as a rust
prohibitor for treating steel frames, although it is listed as a way of
tackling rust stains by a learned architect here in one of his books. You
might be able to both remove the stain and inhibit further rusting in one
fell swoop. Also, as an idea, but not one I have tested, might be to use the
above as a poultice. Attapulgite or Sepiolite clay, when mixed with an
active reagent, applied as a thick paste to the surface of the stone and
covered with clingfilm, allows the solvent to penetrate below the surface
and then, as it dries, draws stains to the surface.

Whatever, wet the stone first and then wash off thoroughly.

Bear in mind though that if the boulder is well weathered, it is likely that
the rusty patina will recur as previously stated. If it is limestone or
something else, I have a few other 'recipes' to hand.

Scott Engering