3M™ General Purpose Adhesive Cleaner

If the adhesive is yellowish brown, it may be a polyurethane sealant, since they tend to turn brown when exposed to UV.

sandpaper is quite effective

I use it to clean up residual adhesives from decals and foam tapes. It will soften glues but it doesn't really remove them without a lot of elbow grease. Have you tried careful application of a heat gun and a sharp plastic scraper?

I did try the sharp scraper but not the heat gun, I'll try that thanks. I think sandpaper is probably what it will take.

Scraping left some big chips in the gel coat underneath the adhesive, so I'll fill in with putty or epoxy. I plan to use a gasket next.

I use 3M adhesive remover frequently and I find it effective if the solvent is left on the adhesive for a time in order to soften it. In this case, the above recommended technique of using a heat gun, then scraping and using the adhesive remover for the final cleanup may be the best solution. Also, toluene works but you MUST wear gloves for skin protection. Toluene evaporates more slowly than some other solvents, so it has more time to work.

The best scrapers I have for these types of jobs I made from lexan or polycarbonate scraps sharpened to a chisel edge.

I think the gasket idea needs further thought.