In a recent tuckpointing project, the walls were stained with mortar at distinct horizontal bands. The only way we could remove the bands was with diluted muriatic acid. These lines occurred at the level of our scaffold planks. We think they were formed by dust or mortar that dropped on the planks and reached the wall. Do you know any way to prevent this from happening?
Although some mortar may have bounced onto the wall during construction, it is more likely that the stains were created during rainstorms. If the scaffold planks against the wall are not removed, rainwater hitting the planks will splash dust and mortar onto the wall. The best way to prevent this from happening is to remove the plank closest to the wall when it's not in use and place it over the adjacent plank at a angle.