How long it will stay clean depends on a couple of factors. Iron and hard water stains from sprinkler water will stay clean as long as we keep the sprinklers from spraying the walls (and we’re working with the irrigation people to arrange that).

But some of the calcium stains are “efflorescence”, which occurs when sunlight heats moisture that’s inside the brick and causes it and the calcium it carries with it to be deposited on the outside where it becomes visible. How long efflorescence stains will stay clean is harder to predict since bricks can absorb water from dew and rain, and bricks are manufactured using a small amount of lime (calcium oxide). Since the walls are over 30 years old we expect much of the original lime has already leached out and the walls it should be many years before we need to do this again. If we follow up by applying a sealant to the walls, they will remain clean longer.