If it has got a patina it may be not a wise move to clean too harshly. Having said that, if it's OK to give it a good scrub have you tried the good old bicarb / vinegar mix? I assume you have also tried steel wool. I recently cleaned my TMX bowl with a load of eggshells I had left over then, when I emptied my TMX into the sink, I scrubbed that out as well using them and the sink came up sparkling. If you are OK using harsher options, perhaps an oven cleaner or BBQ cleaner might do the job for you but you would have to be sure that you clean the tin out thoroughly afterwards.