How do you remove the white film on aggregate concrete left from rain running off the side of schist walls?

Answer 1

A mild acid will remove the film. The film may be from the rock walls or from the concrete itself. Most hardware stores in the US have concrete remover either in the cleaning products or floor tile aisles. A household product that works for me is Lysol brand toilet bowl cleaner. Spread it on, leave it a few minutes, and hose it off. CLR (Calcium - Lime - Rust) remover will do equally well. If you are extremely concerned about the possibility of damaging the concrete, a slightly milder acid is vinegar, which is 5% acetic acid. Buy the big economy size in the canning supplies section of the supermarket. Hope this is helpful to you.