I've been using the same Winpak for no-chill brewing for about four years. Usually when I clean it I rinse it out with water, then soak with PBW or OxiClean, then rinse again and use for the next batch.
My cube now has a brown/tan deposit (beer residue, but not beer stone) building up on the walls of the cube, and it's been there for a while. It hasn't caused any noticeable problems, but I'd like it to go just in case. This film can be scraped/scrubbed off, but both PBW and OxiClean fail to remove it on their own, and I really can't get inside the cube to scrub it.
So, has anyone else noticed a film inside your cube, and how did you clean it? I'm considering soaking in vinegar or double-strength StarSan to see if an acid will strip it, but wanted to see if anyone had any better solution.