Surfing School - Glassing - Fin Box Install
Lay the box on two layers of 6 oz. glass and cut such that each piece extends about 3/4 to 1 inch above the box. Put a piece of 1/2 to 3/4 inch wide masking tape over the finbox slot to ensure no resin leaks in. I seal the tape with a straight edge to make sure it is solidly secure. Tape the board hole cut out about 1/2 inch, overlap the tail with an edge and turn up the upper tape edges as shown in Figure 01 to make sure excess resin drains off.
Mix a colder (not hot) batch of finish resin and saturate the two cloth pieces. Laying the saturated cloth over the hole, push the finbox down into the hole evenly. I sometimes will paint a very thin layer of resin on the foam first, but be very careful the layer is thin or you will not be able to get the resin out and the finbox will not go all the way in.
You need to work the box down until it is about even with the board's bottom, and not slanted. This ensures the fin will be straight up and down when installed. With the cloth sticking up around the box as best you can, squeegee off any excess resin over the back of the board.
You must then wait with a straight edge razor blade until the resin just starts to set. Carefully cut the excess cloth down as close as you can to the top of the box. Remove the tape over the finbox slot and the board is ready for final sanding. Remember that the tape over the slot will need to be redone when you gloss. I use narrow tape for this so you don't have a funny looking un-glossed area around the finbox after glossing.