I use it for anything I want sanitized before placing perishables inside, such as lotion & body butter containers, canning jars/food-storage containers, etc... I also give my lotion-making and canning equipment such as spoons, spatulas, funnels, etc...a dip in the StarSan before using, too......3 minutes soap, then an air-dry (no rinsing). It's amazing how much longer things such as leftovers, etc., last in containers that have been sanitized with StarSan compared to when they've only just been washed well in hot soapy water.
I have a dedicated 5-gallon food-grade bucket that's always filled with a proper dilution of StarSan. It keeps a loooooong time. The moment it starts looking cloudy and/or the pH goes above 3, it's time to make a new batch. The batch I have going in my bucket at the moment is about 1-year old and is still clear with a pH of about 2 and is working great.
Re: the film....is it a visible film, like a white film? In any case, it's probably a good idea to re-wash and re-do in the proper dilution.
Re: any residual amount left in a bottle or keg, I wrote down this little tidbit of info from Charlie Talley (the inventor of StarSan):
"And last but not least is the foam. Since Star San will not kill when the pH goes above 3 any small amount left in a bottle or a keg will not stop the fermentation process and it is safe to drink the beer that is what comes out of the package. That is why Star San has a no rinse claim."
For what it's worth, when I make fresh-squeezed orange juice, my 1/2 gallon glass storage bottle that I've given a bath in the Star San sometimes has a tiny residual drop or two of the StarSan remaining in a corner that's not completely 100% dry yet before I pour my juice in, and all has been well.
IrishLass