I've needed to try and bring back 10 second-hand Cornis kegs covered in labels of all sorts.
First I filled them with 55ºC hot water and with the help of an aluminum scraper (this prevents scratching), was able to remove most of them. I then sprayed them with Oomph, Goo And Stain Remover letting it sit for a while, then used the aluminum scraper to remove all the glue.
All gone, now to tackle the black rubber marks.