There is nothing that can be done for the pitting, it must be replaced.
If you are trying to get it cleaner, try scrubbing it with a paste of Bon-Ami, it is a kitchen sink scouring powder. USE NO OTHER BRAND! Rub and rub, then do it some more. Rinse well or it will leave an icky white film on the car.
It is similar, but not the same. Comet will scratch it, then you really will get to replace the glass. Bon-Ami is the finest in particulate size of scouring powder. DO NOT USE ANY OTHER BRAND! You likely will not like the results if you do.
If the glass is visibly pitted / feels rough then trying to polish it out is an exercise in futility; getting it clear and smooth again would require you to grind the surface down a few thousandths of an inch while keeping the surface optically correct. Not possible by hand - extremely difficult with the right machine.
Replacement glass is available at any auto glass shop - shop around to find the best price. New glass installed should be well under $200
A lot of people don't like RainX, but if you put the stuff on heavy enough and polish it as the directions say, it not only sheds water but seems to fill in those fine sandbalst marks that seem to infect all older windsheilds. Of course it won't get rid them all but any help is good help. By the way the reason some people don't like it is itis supposed to eat up wiper blades, but I haven't noticed it before. Hope this helps.
[This message has been edited by Fixer Up (edited 10-20-2003).]