[header] How to clean windows [title] Clean stubborn stains. [step] Outside windows are especially prone to stubborn stains because they are exposed to hard water runoff, minerals, bird droppings, and elements that can cake on dirt and grime. There are a few methods you can try to remove marks on inside or outside windows : [substeps] Use a mineral deposit removing cleaner, such as clr. 
Dampen a sponge with cleaner and rub at the stains on the windows. Rinse the area with water and proceed with regular cleaning.