Clean inside surface of digitizer after film removal

I have received quite a few digitizers with bubbles under the protective film, which is fine on the outside as it is easy to clean and will be cleaned before collection anyway, but on the inside it is a problem, it leaves a kind of water mark on the inside of the digitizer where the cavity was, I tried to clean one but the marks wouldn't come off easily and it left the inside of the digitizer looking foggy where I had rubbed it, I guess the inside surface is quite soft and easily damaged, I'm not sure if anyone else has had this problem, but does anyone know how I can clean these marks off of the inside of the digitizer without damaging the surface?