Did you try a washcloth? i had one issue and it was a tiny spot but I just had to have it come out. A hot water soak in PBW overnight, then I dumped nearly all the water out and stuck a wet washcloth in...swirled it around and then rested it on the side with the washcloth sitting on the spot. Propped it in such a way that it didn't roll. It worked for me. It was a spot smaller than a dime but I knew I had to get it off the carboy.
No, i didn't try that. I just ended up getting new fermentors. Got a couple of the plastic big mouth bubblers and a 30L speidel fermentor.
I downgraded that plastic carboy for holding RO water for brew day only.