Acetone
I have used acetone, carefully, to remove the residue from letters, numerals, graphics, and tough spots. But, I allow it to do its works for only a few seconds before nutralizing it with water. I know of several boat dealers who use Softscrub to clean boats. Personally, I have used it for at least 15 years, and have had good results. I've also used paint thinner for difficult stains, with reasonably satisfactory success. But, again, don't let it sit on the fiberglass surface for any length of time before cutting it with soap and water. A product called "Colorback," made by Turtlewax, has worked for me for restoring dull, faded painted surfaces.