How do you guys get "stubborn stains" in a carboy?
I use the "clean as you go" method. I'm almost done cleaning by the time I'm done brewing.
You've got to sit down and figure out a process that works for you.
I just spray the carboy out after emptying and it comes out clean. Then I'll use some Oxy and my "rag on a stick" in my drill to "scrub" it and re-rinse. then I'll shake some Star San in it for a few seconds and drain. Done.
I never get built up gunk (see primary for that) in the secondary. For that there's an overnight soak in Oxy, rinse, and a sponge wipe down before air drying.