Remember those blocks of balsa that we installed inside the board by the tail? Those are meant to give structural support to the fin box. To install the fin box we needed to cut a hole in the bottom with a router. We purchased the fin box from Wood Surfboard Supply. The box is plastic and the removable fin itself is glass and can be adjusted fore and aft to fine tune the trim.
If you were thinking ahead you would have carefully marked where the balsa wood blocks were after the bottom was stripped so you would know exactly where you could install the fin box. We forgot to mark where the blocks were, so our hole extended about an inch too far aft and we had to plug the bottom of our hole with more small pieces of balsa.
To get a straight cut, we marked where the fin box hole should be, then set up a simple guide using a 1/2 inch board taped and held firmly against the bottom. Because of the curvature of the board, it was difficult to clamp a guide in place.The first few passes with the router were shallow, just used to define the area. Later we cut it the hole the appropriate depth (a little over 1 inch) using multiple passes, then cleaned it up with a chisel. Be sure to make enough room in the hole for a layer of fiberglass and thickened epoxy in addition to the fin box itself.
A lip about 1/4 inch high surrounds the grove in the fin box. This lip is there to keep epoxy from getting in the fin box during installation, and is meant to be removed after installation of the box. So in order for the fin box to lie flush with the bottom, the box should be installed so that the lip will stick above the surface.
When mounting the fin box it is important to have the fin in place, so you can tell that it is perfectly perpendicular to the surface of the board. Try to avoid getting epoxy on the fin. If you do, it will sand it off easily, but you might want to cover it with tape to keep the epoxy off. Also, mask the area around the box with packing or duct tape to protect it from epoxy. We brushed the inside of the hole with epoxy, put a layer of fiberglass and wet it out, then spread a layer of thickened epoxy in the hole before inserting the fin box.
After the epoxy cured, we removed the fin from the box, cut off the excess fiberglass and epoxy, and used a block plane to carve down the plastic lip of the fin box. Be careful not to mar the board with the block plane. After we carved most of the fin box down, we removed the masking tape on the board and ground down the rest of the box lip flush with the board surface with a random orbital sander.
The fin box kit from Wood Surfboard Supply also comes with a leash cup. It should be mounted on the tail of the board. It holds the leash which keeps the board from floating away from you if you fall off. The leash (sold separately) is attached to your ankle with Velcro. To mount the cup, we drilled a hole 1 inch in diameter in the center of the tail block on the deck, spread thickened epoxy in the hole and pushed the cup in. The area around the hole was masked with tape. After the epoxy cured, we ground down the cup edges flush with the board surface with sandpaper.
Placing a Vent
Simply letting the board sit in the sun for a few minutes will result in an increase in the internal air pressure, as I found out when moved our board from the garage to the yard. After just a few minutes in the sun, opening the vent released a long and dramatic rush of air. Imagine how much the internal pressure rises after letting it sit on top of a hot car for a few hours! I recommend installing a vent to equalize the external and internal pressures, and keeping it open whenever the board is not on the water. The board probably won’t burst at the seams right away if you forget to open it one day, but repeated cycles of high and low pressures will eventually lead to failure.
Ideally, you would have a hatch to drain the interior and allow air to circulate in case water gets inside. This is especially important when the inside of the board is bare wood and not sealed. If the inside gets wet the strips will start to warp and you would notice it as ridges running along the length of the board.
We placed a simple vent (also available from Wood Surfboard Supply) consisting of a screw and a neoprene washer at the tail next to the leash cup. The central spar effectively separates the left and right halves of the board into two separate air chambers so it’s important that the vent overrides the spar. The location of the vent at the tail was chosen entirely for aesthetic reasons. It is actually safer to put the vent at the nose where you can always see it, as a constant reminder to close it before you get on the water, but we felt that that would detract from the look of the nose. You may want to install two vents, at the tail and nose, to allow better air circulation.
Epoxy will discolor, turn brittle, and eventually disintegrate with prolonged exposure to UV radiation. It is important to protect it from sunlight with varnish. We coated ours with 4 layers, wet sanding in between with 300 grit paper. Before varnishing I scrub and rinse the board off to remove any dust and contaminants. I try to work in as clean and dustless area as possible. This can be difficult to create in a workshop after all the cutting and sanding that’s been going on. I vacuum the shop well, wet the floor down and let the dust settle for a couple days.
Prepare the varnish by filtering it through a paint filter into a clean container. Pour only as much as you expect to use. To improve the flow, I dilute it 10% with high quality paint thinner, the kind intended for actual thinning, not for cleaning. Once the can of varnish has been opened, it starts to go bad! To keep it fresh, you need to keep the oxygen off it. I keep a cylinder of BernzOMatic propane handy and blow it into the can as I close the lid to expel all the air out.
I like to use the roll-and-tip method for varnishing. A 1 quart can should be just enough for 4 coats on a board this size. I use thin foam rollers and a high quality brush. You definitely don’t want to use a cheap brush for this, because it will leave loose bristles behind. I pour the varnish into a pan, wet the roller, and roll a layer onto the surface, then I go back and tip it off with the dry brush. The roller gives you a thin, even layer that won’t run. The problem with rolling is that it leaves a lot of tiny bubbles. Tipping off with a dry brush pops all the bubbles. Start from one end of the board, and always work from wet to dry, to the other end of the board. Don’t go back over an area that you’ve already varnished: you will leave brush marks because it has already started to dry. When the varnish has completely dried, wet sand it down with 300 grit paper, preferably somewhere other than where you do your varnishing, and just enough to get rid of any drips and dust particles, then dry it, wipe off all the dust and repeat another layer, until the can is gone.