Background

Iron or rust stains, as they are commonly called, are usually yellow to deep brown. They are more noticeable on cream or light coloured bricks.
Causes

‘Acid burn’ or rust stains can be caused by insufficient pre-wetting of the wall prior to applying hydrochloric acid which is too strong, causing the acid to react with the iron oxides in bricks or mortar.

Staining can also be caused by rusting iron or steel such as lintels, nails and bolts, railings and packaging straps.

Regular wetting from bore water used for irrigation and by welding spatter from welding near brickwork, may also cause iron staining.
Remedy

Brush or spray RidRust to the dry wall, preferably in the sun. The iron stain will normally disappear in 30 minutes to 24 hours. Wash off treated area. Clay masonry should be neutralised with Neutril which should not be washed off.

The phosphoric acid in RidRust will tend to bleach any iron oxides used as colouring pigments. For this reason pigmented mortars will fade, so the entire wall will need to be treated to maintain a uniform appearance. Alternatively keep the solution off the mortar.

In extreme cases a stronger solution of phosphoric acid or a poultice treatment may need to be used.