To get the best results and beautiful rust coating, weathering steel requires an alternating cycle of wet and dry conditions. Moisture helps to create the oxide layer. As it dries, the oxide layer starts to dehydrate, resulting in a compact adherent layer with low-permeability: the protective patina. During patina formation, some of the oxides are washed out by rain. The amount of oxide leached out by the rain diminishes over time, but never stops completely. This can stain neighbouring materials. Careful design of the facade is needed to ensure that the oxidised rainwater is collected and directed away from other materials to eliminate staining.
