Our organization received 8 retired police bicycles that we will be refurbishing and turning into loaners. Some of them have been sitting in the Arizona sun for some time and the "Police" decals applied to the frame have been baked on and are cracking. Peeling off is proving nearly impossible as the sticker doesn't have the structural integrity to maintain a nice peel with all the cracking (looks like crazed paint). The decals are definitely not clearcoated over (the frames are Fuji and the Fuji logo certainly is, but the Police decals are applied on top of the paint).
I have tried a scotchbrite sponge with the following:
The problem with more aggressive solvents such as Acetone (nail polish remover) and Zylene is that they will likely eat into the paint.
Typically the PSA adhesive used with stickers is acyrlic based, which means that acetone is the right solvent. The problem is that the sticker is usually PET, which will does not disolve well in acetone.
My recommendation is: Extra sharp razor blade followed by Acetone to remove the residue, followed by citrus bases solvent for final clean up.