Save your time and effort and just buy a new shell! Even after removing it, some of the paint may have stained the lexan or there may be score marks in it etc..
But....if you do want to, then brake fluid works. Pour some in it and soak some paper towels in it and put them in the shell. Leave it 20 mins then scrub and clean. Repeat as needed.
Did I mention just buying a new shell?
