Brown staining - Brown streaks down front of render

[fidaleo]

Hi

Had some rendering work done on walls in garden in limestone white

They have been topped off with Indian stone. During periods of heavy rain I seem to get a brown streak down the render - only in one place at the moment. Is this related to the mortar used to lay the stone on the top of the wall or the stone itself.

Tried scrubbing with water/soap but doesn't shift it

Any advice would be appreciated on what I can do to reduce/remove the staining

Wondering if it might be Manganese staining ?
Do you think its worth using a solution of oxalic acid to try and remove the stain

Regards

Martyn

[Tony McC]

Could you get a photie for us?

Render and walls aren't my field of expertise, but we do have some under-utilised brickies and the like lurking around the Brew Cabin.