What Causes Water Stains on Side Glass? Water itself is rarely the problem. The issue comes from what remains behind after water evaporates. Common sources include: As moisture dries, minerals such as calcium and magnesium remain on the glass surface. Over time, repeated exposure creates: The longer these deposits remain, the more difficult they become to remove. Heat and direct sunlight accelerate the process significantly. Start by Identifying the Severity Not all water stains are the same. Before choosing a cleaning method, inspect the glass carefully. Light Water Spots These appear as faint rings or light spotting visible only in…