did you put any cloth on the fins before you attached them to the board?
i usually put at least two layers of roving on the base of each fin. There's about 20 strands in each layer of roving. The use 2 layers of 4oz on each side of the fin, up the full rake, and out onto the board about 3", but trimmed in an arc like this - ) . Reason being, square edges are harder to sand out cleanly. I was taught to cut all the pieces in advance, in a shape like an american football, or pill etc. and then lay it up, trim when gelled, and hotcoat.
I have put one layer of 4oz on before, but developed cracking along the base, and the fins flexed too much, and washed out a lot.
Anyway, hope that helps.
learn the rules, so you can break them properly