I know of no solvent that will remove 5200 once it has hardened that won''t also damage4 the fiberglass. I also question the use of Starboard in a structural application such as a rub rail and also I question using a permanent adhessive like 5200 to attach a rubrail since rubrails are prone to damage and need to be removed for
repairs with enough frequence that they should be caulked with a removable caulking.
Jeff