First, I'd check and make sure all the bolts are removed. I know on mine, the bolts for the rear fin are shared with the ski pylon.
If all the bolts are out; here's how i would approach it:
5200 is pliable, like rubber; it is not brittle like concrete. Banging on it with a hammer likely won't do you any good because the glue just stretches and bounces back. Also, using a chisel will subject you to the same bounce-back and leave open the possibility of tearing up the gelcoat with the chisel. To break the bond of the glue, a static force must be applied to SLOWLY pull the glue apart.
Here's how I would do it. Get the biggest/longest crescent wrench or pair of vice-grips that you can find. Orient the wrench up/down with the jaws clamped to the bottom of the fin. This up/down orientation along with gripping the fin at the bottom gives you the most leverage.
Take your time here and start working the fin side-to-side. It won't move a whole lot at first, but you should feel the 5200 flexing as you move it back and forth. As you work it back and forth, slowly start applying more pressure to your wrench. You should feel the glue start to flex more and more. Keep on going, patiently, and eventually one side should open up and you'll be able to see the glue stretching and coming apart. Just take it slow and you should be able to break the glue apart without it pulling the gelcoat off with it. There will be some remains of the glue on the hull, and you'll want to scrape that off with a razor blade.
If you were to pull some of the gelcoat off, its still not a huge deal. You would only lose the gelcoat NOT the structural fiberglass beneath it. You can just apply an epoxy barrier coat to the damaged area to keep water out of the fiberglass. Then you just cake on some more 4200/5200 and bolt your new fin on.
Salty, I have to respectfully disagree with the notion that 5200 won't come off without taking part of the surface with it. I've been told at the boat store:"4200 if you want it to come off again, and 5200 if you never want it off," but i have my doubts and it could very well be just an old seadog's tale.
I have removed and remounted or replaces nearly every underwater part of my boat. While I don't know what adhesive was used at the factory, I can say that I have only used 5200 when doing repairs and I have yet to damage the gelcoat when removing those parts with the 5200 on them. Some did require a fair amount of coercion, but not, in my opinion, an unreasonable amount.
A side note: when speaking with a SC rep at the factory when i replaced my driveshaft strut; I asked him whether i should use 4200 or 5200 because I was still going on the presumption that 5200 is completely permanent. He recommended using 5200 on ANYTHING below the waterline.
Thats my $0.02. I am open to criticism however, and would like to hear your experiences with 5200.
Oh yeah, OUI: don't forget to check to make sure all the bolts are out.
DKJ
1992 Supra Mariah - Red
PCM 351HO 285hp - PCM 1.23:1 Transmission
OJ XMP 4 Blade CNC 13x15.5RH
"People do weird things to boats" -Unknown