Of course, if the windshield is very stained, with dirt, the first thing you have to do is remove the brushes (do not forget to clean them with a brush or cloth), dump a bucket with water and detergent so that the dirt comes off.
Then you should pass a soft bristle brush, from top to bottom, so that the liquid is removed by the windshield drain. Only when you rinse it, can you apply a household glass cleaning product.