i had customers that ruined their case paintjobs
trying to remove coffee stains with strong solvents
when all it needed was some plain water

for every stain or residue there is always a right cleaner
but because we can't know in advance
its better to proceed gradually and with caution

you should start off with plain warm soap-water
if it persists, move on to something stronger like a glass cleaner
then alcohol
then gasoline-petroleum
then lighter fluid
and when you want to clean a bare metal surface you may proceed to acetone
then nitrus solvent
then graffiti remover sprays

if its still there...
get sourself a sander and bring it down to the ground...