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Film on the windshield usually comes from the plastics/vinyls "gassing" and will lessen with the age of the vehicle.
You will never get rid of it completely, but several products can help with it. Namely a good glass cleaner like Stoners or Sprayway. Keep the dash protected (with 303 Protectant ) may not help with outgassing specifically, but will reduce harm from UV rays.
just have to add -- I just used Stoners for the first time on my glass and it DID make a noticeable difference. I've always kept my windows scrupulously clean, and the past few months was using NXT window cleaner.
I've never seen glass look so clear as after this one use of Stoner's -- I'm hooked.
I am also a Stoner convert. It is so easy to get great results. I have found that on severely neglected glass with a thick film, you may need to do 2-3 applications before you really get it good and clean.
Nothing will totally stop it but definetly use Stoners to remove it. I usually don't like to use anything but a damp towel on my dash and door panels on my vette unless I have to.
Nothing will stop the migration of the platicizers out of the dash... it is driven by a basic law of Physics called gradient diffusion...
Things flow from areas of high concentration ( within the plastic) to areas of low concentration (the air) these volatile vapors sublime (return to solid state)when they hit the glass.
a protectant like those mentioned provides a molecular coating that creates a boundary layer to slow the migration.
A combination of protectant ( use whichever gives you the gloss level you want) combined with a good window cleaner is unfortunately the best band-aid avaliable.