There are available quite a number of liquid coatings that can do double-duty protection of your windshield: fending off precipitation and to a certain extent bugs and dust; and preventing or repairing acid rain and other chemical action.
For rain dissipation: the most common such products cover the windshield with a fine, transparent layer of a polymer which bonds to the glass and will prevent water from accumulating in layers or sheets. Instead, drops will form which will be blown off the windshield by the airflow as you drive or swept off by the wipers. Another type seals the microscopic pores in the windshield which would otherwise serve as anchor points for the water sheets; this type won't cause the water to form drops so well but the sheets will be much easier for the wipers to clear off. Most of these products are applied when the windshield is dry and clean, but there's at least one type that you spray on when it's actually raining; the wipers then serve to spread the fluid where it's most needed. Obviously, this won't cover the entire windshield but it will certainly help. The polymer products will help keep dust or bugs from accumulating but won't stop them; they will also prolong wiper life as they provide a dry-lubricant effect.
Note, however, that these products will not make up for poor wipers. Treat them as supplements, not substitutes.
There is at least one product that protects against and claims to reverse acid rain damage. It's a three-part process: first you prep the windshield with a spray which gets into all the pits and cracks and reams them out; this is a power-spray process. This step is optional but is the one which removes acid-rain damage. Then you use the "Kaolin clay" to clean the glass: this stuff really catches everything! (But don't be too impressed: kaolin is the "proper" name for clay!) Wash down, clean and buff the windshield. Finally, you apply the shielding polymer to the windshield.
Most of these products claim an effective life of a year or so. It's doubtful that they will last as long in northern climes, as scraping the windshield has got to wear the polymer down more quickly than normal wear and tear.
These costings don't cost much, so you won't be out of pocket much if they don't work; but as you know, a wet, oily windshield is difficult to see thorough, so they're definitely worth trying out.