We are homeowners and are wanting to paint a garden wall. It had a lot of green mould, which we have successfully removed with a pressure washer, but there remains a white substance which resists pressure washing and even scrubbing with a metal-bristled brush. We thought it might be efflorescence but I understand that should dissolve in water.
Any idea what it is? Do we need to remove it before painting, and if so, do you have any suggestions on how to remove it?