ok, the guy at the shop said yeah varnish is the way to go.
I chiselled slits for the fins (made from the 6mm ply) and stuck them in with glue. They seem pretty strong, but if they do happen to break off i'll just rip them all off and make do without them. I probably should've put the fins closer to the ends of the board, but this will do
I also got some foot pads and some NSI straps. I drilled holes (in a position so that my heels are a bit closer to one edge than my toes are to the other), and an extra wide bit on the bottom of the board on each hole so that the bolts I put through are countersunk. (I'm putting the bolts through from the bottom up, so washers/nuts go on the top). Glued the bolts in.
Gave it a first coat of varnish (painted the bottom and fins, then put it resting on it's fins on some baking paper and painted the top). Will lightly sand and give it a few more coats.
I'm pretty happy with the way it's looking
Oh when I was at the shop I also picked up a board very similar in size to this - at it definately wasn't any lighter than mine, so i'm not so worried about that now.