The answer to tough stains on window cleaning is to first try a DIY solution using a vinegar-based window cleaner or a specialized window cleaning solution. Common causes of tough stains include mineral buildup, hard water deposits, and grease or grime. Symptoms include streaks, haze, and spots that won't come off with regular cleaning.
If the DIY solutions don't work, it's best to call in a professional window cleaning service. They have access to more powerful cleaning agents and tools that can effectively remove stubborn stains without damaging the windows. A professional will also be able to determine the root cause of the stains and provide recommendations to prevent them from recurring.