Although I'm hesitant to suggest it , perhaps a pic might give a clue about what you mean by 'grubby' .
Sometimes glazes on such items have been damaged by decades of scrubbing with abrasive cleaners , and stains penetrate into the porcelain underneath - in such cases , the porcelain will remain porous & stains will constantly reappear ..... reglazing a funky old toilet might be just a crappy idea .
The words 'flower planter' come to mind ....
Plenty of OTC cleaners work , depending on what you're attempting to clean : CLR for lime deposits , bleach , vinegar , abrasive clensers & etc .
It's true that 'cola' is acidic , but it does contain coloring agents that just may stain your Thomas A. Crapper further .
To see evidence of the action of colas' Ph values , put a little of it on some aluminum foil & let it sit overnight .
