I was trained with salon system and have never used film, have no problems. You can leave it on up to 15mins but remember if you put the perm on and miss an area, then applying fix won’t do anything to it, vice versa, they both have to be exactly the same coverage for exactly the same time.
I did notice a batch recently that was a bit lumpy, overly thick. Must have been gone off as it didn’t do much but thankfully it was on my own lashes! I often use the entire sachet aswell. I don’t do enough of them to do half a packet at certain times of year and I’d rather my ladies got an amazing lift every time than save solution if there’s not another one coming in for it within a day or so.