I find a 70p bottle of surgical spirit gets rid of most thangs, used to use it to clean up old traded in PC kit, which would have all kinds of cr*p on it, from scribbles to stickers..
If not, try a photographic suppliers for Arclone, my first employers yonks ago where into doing very odd stuff with photography, but not snap shots, techy stuff, but Arclone was used to clean anything and everything around film, but we found it'd dissolve just about any muck or stain... Watch out, it reeks, and U WILL get very high using it LOL, also WASH it off anything it comes into contact with, or it may well dissolve that as well, used to make mincemeat of surgical gloves with it.