Wow, looks like you’ve got a serious case of “Ring Around the Carboy.” Thanks for sending in the picture, I always love it when readers do that because, especially in cases like these, it gives me a good visual to go from. I can see that pesky stain on the inside of the neck of your carboy. One Step is a great cleaner but I’m not so sure it’s strong enough for your purposes here, i.e. it won’t really help you get that film off.
If I were you, I’d make sure you had a stiff-bristled “carboy brush.” If your local homebrewing/fermentation supply store doesn’t have one, you can find one online by entering those terms into your search engine of choice. You want a brush with an angled handle and/or an angled brush, so you can get into every curve and corner of your fermentation vessels.
Rather than use One Step again, I’d use something that’s a stronger base, which will help to break up the residue around the neck area. A classic that I love is peroxycarb, which is the trade name for a basic (high pH) powder of sodium carbonate and sodium percarbonate, which also releases hydrogen peroxide when added to water. I suggest an overnight soak in a strong (about ¼– ½ cup powder to a gallon of water/16–32 mL powder to a liter of water) peroxycarb solution, followed by a hard scrub with your angled brush and then a rinse with acidulated water (like citric). If that doesn’t work you could try a little abrasive cleanser like Bar Keeper’s Friend or even Comet cleanser. Just don’t use anything with bleach! Always follow any basic cleaner with an acidulated water rinse, and then a regular water rinse, to make sure you’ve removed anything that might change a wine’s pH. You definitely don’t want any basic residue hanging around inside your carboy.
I’ve got a pretty good feeling that with an overnight peroxycarb soak you should be able to scrub off that film. If not then it might just be a permanent stain on the inside of your vessel. If a strong base solution won’t take it off and you’ve scrubbed up and down I’d just live with it. It’s unlikely that at that point it will dissolve off into your wine!
