Cause
This staining is caused by iron oxide being present in the sand content of the stucco finish mix and is activated by moisture coming into contact with oxide particles.
Ensure the surface is completely dry and apply a suitable sealer to the affected areas which would normally be oil or spirit based. If a standard water based masonry finish is to be applied and a stabilising solution has been used and the finish is glossy glossy finish then this should abraded to provide a suitable key.
Where rust staining is present it is recommended to use an oil based masonry paint based on Pliolite® (Goodyear Tyre & Rubber Co. Inc) resin.