How do I seal between glass and gelcoated fiberglass on a boat?

If there is constant UV exposure, then I would suggest one of our marine sealants like 3M™ Marine Adhesive Sealant 3000 UV or 3M™ Marine Adhesive Sealant 4000 UV. Both have excellent UV stability and high bond strength as long as the substrates are clean and free of any contamination. Proper surface preparation is a key to bond strength. We recommend solvent cleaning with 3M™ Adhesive Remover or methyl ethyl ketone (MEK). Do not use isopropyl alcohol to clean when using an adhesive sealant. For increased bond strength, you can abrade before cleaning. There are also primer options available that can be used after cleaning.