The answer to tough stains on windows is to use a combination of cleaning methods and products to effectively remove them. Common causes of tough window stains include hard water buildup, grease and grime from the environment, and dried-on debris.
For DIY stain removal, try using a vinegar-based window cleaner or a baking soda and water paste. Apply the solution, let it sit for a few minutes, then scrub the stain with a soft-bristle brush or scrub pad. Avoid using abrasive cleaners or tools, as they can scratch the glass. If the stains persist, you may need to call a professional window cleaning service to deep clean the windows using specialized techniques and equipment.