You can use Acetone
Bill:I had a similar problem finding and repairing a leak on my Rub Rail (1987-H28.5). I used paint thinner at first, but found Acetone to be the best at removing the original caulk without damage to the fiberglass. I also used an unorthodox procedure to build up a small cracked area under the rail until I could replace the entire rail...Liquid Nails. You can shape Liquid Nails and it bonds to fiberglass and doesn't absorb water...it's easily removed from fiberglass. I would replace the caulk with 3M 5200 or better caulk. 3M makes the best caulk for Rails and Windows. You can purchase Acetone at Lowes, Menards or Home Depot at a considerable savings over West Marine.Good Luck...Dan