If you still think there’s paint or tar or something like that on your windshield you could try to use acetone (aka fingernail polish remover/slightly weaker) to remove the paint. However be very careful not to get it anyplace else other than the windshield; because it could damage the paint to your car, weatherstripping, plastics, etc…. If the outer layer of your windshield is not glass it could damage your windshield. So you might want to test a small area that is outside of your normal field of view; before trying the affected area or the entire windshield. If there is a lot, it might smear at first so you may need to repeat with clean paper towels. Also beware that acetone and its fumes can be flammable and/or explosive. When you’re done completely dry. Then you ought to clean using a window cleaner to dilute or remove any additional residue.
Then you’ll probably want a coke treatment. ( I think most sodas work, but I think regular caffeinated coke works the best). There could be a film of petroleum solvents or oils on your windshield that can make it look foggy and can do weird things combined with moisture to reduce your visibility. I think there’s a little bit of acid and phosphates or something like that in coke that help break and remove that film. I prefer to do it when it’s raining or just before I take it through the car wash. A dump a quarter cup or so of coke on the windshield and then towel it around the entire windshield. A usually repeat once. Then you’re going to want to get the coke off; because you don’t want to dry on the windshield or to dry on the paint or to attack any metals or paint. So it’s best to hose off afterwards, or to take through a car wash, or to drive it in the rain.
Sometimes silicone wiper blades or an excessively heavy or old coat of rain away type of products can cause a weird greasy film that can obstruct your view. So sometimes the Coke treatment about monthly will help prevent that building up. Sometimes it’s necessary to replace the premium silicone wiper blades, with cheaper rubber or neoprene type of wiper blades that don’t last as long.
As twelvepole indicated if you do have something like paint on your windshield it may take several repeated applications of paint remover or acetone with a new towel on each application to remove it. Then you will likely have a petroleum-based film on your windshield that you’ll probably need something like Coke to remove it.
Sometimes you'll have a nasty film on your windshield and it won't be apparent until the light hits it at the right angle, or until you get out in the rain or snow or at night.
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