Sonax Glass Polish is a product that holds most of its value in deep efficient cleaning of automotive glass.

Products like these often not only carry some mild polishing abrasives (like regular polishes), but also have cleaners that more effectively (eliminate polish lubricants) make wipe off easier but also have cleaners tailor made for bonded contamination.

Just like clay, a glass polish can address problems (especially windshield) that traditional washing cannot.

Normally for a complete and thorough prep of automotive glass, I would recommend a clay and polish.  Picking one, I would say a minimum of clay.  If it is a dedicated glass polish, I could make the case of going straight to a glass polish if I had to pick one or the other.  The reason being a glass polish has dedicated cleaners with polishing abrasives that provide a comprehensive cleaning of the glass.  The products are tailored made for removing bonded road/environmental film and light waterspots that become stuck to glass.

One of the first of many vehicles I evaluated this product on was a truck, that has seen few washes and all of its time outdoors.

First application slightly heavy application.

Working in circular strokes for about 10 seconds I worked the product in with my working area being half of a windshield.

Second application I used this much.

With half of the windshield per application, using this amount, the ease of removal with a microfiber towel was apparent.  It was such a breeze to use, that I consider it a good multi monthly secondary step window cleaning option.

Below is a description of the dirt that came off of the windshield after washing the car.  It speaks to one of the reasons why you may struggle cleaning glass on a neglected vehicle.  A thorough effort with a good glass cleaner may have addressed this.  A good effort with a clay bar likely would have addressed it.  There is also a chance a clay bar may struggle or need work with road film on windshields.  Smearing grime is the source of many detailers frustration with cleaning glass.

Although I moderately worked it in by hand, machine polishing application is an option.  Machine polishing would be necessary for very minor scratch removal or removing severe water spots.  Although by hand with moderate effort, this glass cleaned up incredibly well.