First up, apologises if this is an old idea. This is a job I did a month ago, and decided to share it with you all.
The mini-tuttle side fin on my 2018 JP 79 FSW board is malformed; It has not been cast properly. I've owned this board for a while now, but I finally had enough of a sloppy starboard side fin no matter how tight I did fin up. So, I decided to repair. As I was preparing the board, a friend dropped over. He is a model aeroplane enthusiast, and flies them competitively on a web site that looks very similar to GPSTC. He asked me what I was doing, and I told him the plan. He said: No, Don't do that, do this !. And I thought: Wow that's genius !
The "Before" pics:
The idea is to recast the fin box. You use the fin as the mound. You seal up the fin box and the screw hole in the fin. I used blue tack and some electrical tape to seal these holes. A non-bonding agent is applied to the fin. Yes, that's the one, Vaseline ! (and I know many readers will be very well familiar/conversed with this product). And so the fin is liberally coated with this "non-bonding" agent.
The board needs to be covered and taped over to prevent accidental spillage of the glue. I also applied the non-bonding agent to the board surface in case glue does end up there.
Mix some of your flavourite marine grade epoxy (3-5ml). About 1-2ml is put into the (now sealed and coated) box using a syringe. The fin is then carefully placed into board and pushed down hard. If not enough glue is applied, carefully remove fin, apply another 1-2ml and repeat. You don't want to apply too much glue as it will overflow and make a mess. I would say it's better to be under than over IMO.
Nb/ I used a square to ensure fin was square with board.
The result (tape and blue tack not removed yet, and job not cleaned up):
Even though my friend said this technique is "sure fire. You wait and see !", 4 hours after application, just as epoxy turned to plastic, I removed fin just to make sure it could be removed. It popped out easy. Pewh !. I then put it back in and let it cure for 12 hours. The next day, after curing, I cleaned fin box up with some paper towels and a file. I was back on the water that afternoon with a perfectly fitting fin. "Quick 'n easy" 
J
Done very similar many times as the old MTs are weak and a hit to the fin often punches the rear thru the box. To avoid box replacement (if its a minor crack) I use epoxy, milled fibres and black pigment then do as above..... just in the cracked area.
Better mould release is shoe polish (Nugget, Kiwi) if not buying the real stuff. You can buff it off flat to avoid the stippled finish the Vas has given you (also a wax that dries can't move like the Vas can)
Thanks Mark. Will try the shoe polish tip next time
What do you use for black pigment ?
Black epoxy pigment :)
Proper stuff from fibreglass supplies is best ..... but the cheap crap ones on eBay where you get 10 different colours for $15 or so work OK
Has to be the epoxy one, the ones for fibreglass resin and PU ones are different
I have used ground activated charcoal, the kind in water filters, to color epoxy for surface repairs on a carbon wing, worked fine.
I had a piece of metal that needed soldering and ran into this high heat resistant metal epoxy. Great stuff, would work here I think. It's a two part product with some fibers.
