What does off-gassing film mean ?
Others have chimed in with answers--interior plastics giving off oily gasses that adhere to the glass. Then there's the whatnot...cigarette smoke, environmental pollutants, even what you may be exhaling into it even if you don't smoke.
What it boils down to is whether interior of the windshield lights up with smears and swirls when the sun hits it just right. Conventional glass cleaners mostly just smear it around. Auto glass cleaners are not any better.
If that is the issue, the next question is how thick that residue might be. If the vehicle is fairly new one of the suggestions above might work. If there are several years of residue that have just been smeared around with glass cleaner the problem is more difficult.
For particularly stubborn residue, you'll find a couple of youtube videos where detailers use alcohol or other liquids along with Magic Erasers. The erasers have an ultra fine abrasive that should not damage the glass. That's quite labor intensive and it might not work for you. It didn't for me.
As a last resort, you could try Nu Finish car polish, my personal hack that I've seen nobody else on youtube or elsewhere recommend for interior. This polish is assumed to also contain ultra fine abrasives. I had been using it on the exterior windshield based on a youtube video for beading and smoother blade glide with good success then figured it couldn't hurt on the inside when prepping a 14 year old car for trade.
Position the car where the sun lights up all the smears, then easy on the Nu Finish with a damp microfiber, easy off with a dry one, two treatments for especially stubborn film. That solves about 90% of the problem if you later see in a different light some smears you missed. Then just buff out those residuals with a dry microfiber and you're good to go. I've been doing this a for a couple of years now, repeating with my annual detail, on two other cars since that trade old trade with good results.
Further responses below may be outraged. "Car polish on glass!? No way!" Well, you may want to avoid using it on tinted windows, especially aftermarket tints. I did use it on the rear factory tints on that 14 year old trade without any immediate issues but not on other vehicles to err on the side of caution. Other than that, if all else fails, Nu Finish is an option.
