We renovated an old Scamp and I found the best stuff to remove all the old "scum", that was supposed to be sealant, was carburetor cleaner.
I can't verify what this would do to new gel-coat because the finish on the Scamp was shot and we were prepping for painting so I wasn't real concerned with anything left behind except a perfectly clean surface. It dries really fast so I would shoot the carb cleaner onto a paper towel and wipe the glue "yuck" quickly with extreme effectiveness. On particularly tough spots that were thicker or harder to wipe off I'd spray directly onto the shell and it worked very well removing lots of extraneous smears and slimy yuck.
If I was going to try this on my new fiberglass trailer I'd pick a hidden spot and test it before I went wild.
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