it think it depends on how much elbow grease your willing to apply.
there is a few things you can use, they say baking soda rubbed on will do it, or a vinegar and water solution.
iirc carb cleaner will make a good enuf job of it, but once again you would really need to give it a brush in.
id try gunk too, it might shift it, leave it overnight and wash it off in the morn.
i havnt tried any of them tbh, so id be doing a bit where it wont be seen first to see what would work best for you.
paint stripper might work too.
some acids will eat into it and do it more harm than good so be carefull.
mightnt be a great answer for you but some of them might be worth trying, let us know how you get on.