To prevent staining, it’s important to understand the process by which it occurs. In order for stainless steel to remain free of rust, it must be exposed continuously to oxygen for it to replenish and maintain the tough oxide layer that resists corrosion. Stainless steel used on deck is exposed to air, which certainly provides ample oxygen to maintain the oxide layer. Even stainless steel used below the waterline, provided that the water moves freely and regularly past it, may be exposed to enough oxygen to remain passive. If, however, the stainless steel is exposed exclusively to an oxygen-poor environment—stagnant water, for instance—then it’s likely to enter an active state.