Most stains and discoloration can be removed from brickwork if the proper cleaning technique is employed. There are essentially two categories of stains; those which are externally applied to the wall and those which originate from within the wall. Those which come from within a wall may need additional investigation to prevent the stain from returning.
As always, it is important to thoroughly saturate a brick wall before application of any cleaning solution. As a general rule, acidic cleaning solutions should only be applied to red brick with no surface finish such as sand. Also, an inconspicuous area of the wall should be tested with any cleaning solution for compatibility prior to application on the entire wall. Any cleaning solutions should be thoroughly rinsed from the wall.
Most stains can be dealt with by thoroughly washing the wall with a common household or kitchen cleanser dissolved in water and applied to the wall with a stiff bristle brush. If this is ineffective, a poultice which dissolves the stain and pulls it into an inert material may be necessary. The inert material can be talc, whiting or fuller's earth while the solvent will vary based on the type of stain. Proprietary cleaning agents can also be employed to remove specific stains.
Sandblasting and pressure washing brickwork can also be options for certain brick w hen especially stubborn mortar or externally applied stains are involved. Bricks with coatings such as sand or slurry finishes should not be cleaned in this manner. Sandblasting and pressure washing should usually only be undertaken by a competent professional with experience. If improperly executed, either of these methods can permanently damage the brick.
For more information on stain removal, see Technical Notes 20.