There have been many questions about BMW windshields, why they get pitted, and why they are difficult to clean. BMW issued a service bulletin about the wipers and what can be done to clean the windshield and make the wipers work more effectively. I have added some thoughts on what I believe can alleviate the problem, and I've also included the service bulletin.
BMW windshields are made from soft safety glass. Due to the nature of the coating on the outside of the windshield, they are prone to "sandblasting". Sandblasting is caused by the debris that is kicked up into the windshield from cars at highway speeds. Eventually, the windshield becomes pitted and is more difficult to see through, particularly on very sunny days. As well, another problem is that on rainy days, the windshield wipers can fail to work properly due to the dirt deposits that can develop.
There is nothing that can be done about the sandblasting problem except to replace the windshield (and stop tailgating big trucks!) I have always used Sekurit and Sigla glass straight from the manufacturer, although BMW has used an American made HBI glass. All of these windshields suffer from the same problem, so this isn't based on the country of manufacture. Eventually, in all cases, the windshield needs to be replaced.
However, to solve the accumulation of dirt on the windshield, BMW recommends the use of Bon Ami to properly clean the windshield. I would also recommend replacing the original blades with Bosch Microedge blades, which I believe leave less residue on the windshield. They are a proven product, are significantly easier to replace since they come with the entire metal arm, and are less expensive to replace than the original blades. Due to the poor quality of windshield fluid available in the USA, I strongly recommend the use of P21S Windshield Boost (or Wurth's if you can find it) where you add a small amount to the existing fluid reservoir. I have found this to be instrumental in helping to keep the windshield clean, even more than Bon Ami, since you use it when you need it - when you're driving down the road.
I strongly discourage anyone from using Rain-X as a substitute for using windshield wipers. My experiences with this product teach me that it works beautifully for a short period of time, but when the Rain-X wears off (within a couple weeks), the windshield becomes downright dangerous. I don't care to spend every week re-Rain-X-ing my cars' windshields. If you have the time to do this, get a life.
Please also note that in 5-series BMWs from 1989 and 3-series BMWs from 1991/2, the windshield is an integral component in the car's body shell. Proper installation is an important safety issue, since the windshield is designed to affect the car's rigidity. The older E28 and E30 BMW windshields are significantly less expensive. Please use a reputable shop and factory glass when replacing a BMW windshield.
Bulletin Number: 61 02 94 (3990)
Noisy Wipers, Poor Cleaning: Group 61 Electrical
Woodcliff Lake, NJ
Windshield Wipers Are Noisy Or Do Not Clean Properly
Windshield wiper blades may start to make noise, or fail to clean the windshield properly, even though the blades are not worn and are properly adjusted.
This can be caused by contaminants on the glass, such as oily road deposits or wax residues from car washes. These contaminants can cause the blades to chatter across the glass or fail to wipe cleanly.
Common contaminants that are not removed by conventional glass cleaner can be cleaned with Bon Ami(R), a household cleaning product.
The cleaner should be applied on a wet cloth or sponge and the windshield must be thoroughly and evenly scrubbed. Wipe off residue with a clean,dry cloth before the product dries on the glass.
There appear to be some imitation products, or lesser versions of Bon Ami. Make sure you are buying the real product. Faultless Starch Company has the rights to sell it in the USA. Up in Canada, it's sold by the S.C. Johnson Company. If you have other questions for them, call them directly at:
Faultless Starch/Bon Ami Company
Kansas City, MO 64101-1200
+1 816 472 4987
+1 800 821 5565
Now you can always go visit their web site, but there isn't much to it yet. For those that need a link, check out this one.