if your windshield has tiny speckles in it as if it was "sandblasted" then I think you need to get a new windshield. I have this problem....the fact that my car was used primarily on the highways by its previous owner created alot of small defects in the glass. So in direct sunlight I see a nice rainbow effect
Windex and any conventional cleaners will not fill in those speckles. I asked this forum how to fix the sandblasted windshield problem, and someone mentioned using a glass polish with a orbital buffer....I haven't done that yet. But you could try that.