Surfing School - Repairing - Fin Repair
Fin repair is nearly as complicated as putting a fin on the board during glassing. If the old fin is cracked along its base, it must be removed by sanding. Use a 7 inch sanding disk, and sand straight down on the resin area around each fin base until the fin can be remove with a slight tap. There will likely be a hole in the foam under the fin, and this is to be filled the same as any other puncture ding.
When the cabosil filler is cured in the fin hole (see Figure 01 picture), sand the area around the fin smooth and flat. Also sand the fin sides and bottom to remove excess resin buildup. Position the fin on the board using non-hardening resin to tack the bottom down. Put masking tape around the board's tail and fin area, flaring at the bottom, to allow excess resin to drip off.
Cut cloth and rope as was described under glassing fins while the tacking resin dries. Remove the tape, and using your thumbs, complete the glassing as was described previously. Squeegee any excess resin off the rear of the board. When the resin has started to harden, remove the tape and use a razor blade to cut the excess cloth from around the fin and excess rope from the front and rear of the fin. Re-tape the board at approximately the same location and paint a sanding coat over the entire area. The picture shown in Figure 02 is a reinstalled fin ready for sanding with the tape removed.
Be extremely careful sanding the base of a re-installed fin as it is very easy to sand through the bottom cloth of a finished board. Try to stay on the flat of the sanding pad rather then use the pad's edge until you are very comfortable with using the sander. A low speed sander might be a good idea for beginners. Most of the final sanding should be done by hand, especially on the rails and fin area.
Glassed on Fin Repair (Splits and Re-Building)
Split fins are basically fins that have suffered massive delaminations between cloth layers. If the delamination is bad, sand the base of the fin until the cut is just flush with the fin's base and then tap the fin off. Be careful not to try removing the fin before completely cut at the base or you will damage your board. If the split isn't bad, the following might work.
Work some resin set to go off slowly into the delaminated section using a syringe with a large gauge needle if available. Get two short pieces of 4 inch wide by 1/2-3/4 inch thick pine and press the fin together using two C clamps, one on each side. Put masking tape and/or wax paper on the wood boards, especially if you plan to use them again. This also helps the surface cure better. Once the resin sets, you will need to sand the fin down, as it will be thicker than before. You will probably also need to build the fin up in places again after sanding as any internal air bubbles will leave holes.
If the fin's tip is broken, it can be built up again by using this method in combination with the approach used to replace a fin. Except in this case, build the body up with small pieces of cloth. Make sure the cloth is fully saturated and also make sure to color the resin similar to the color of the fin or it will look bad. Also, try to press several layers of cloth between the large outer layers or the fin will be mostly resin, and will break off again easily. The harder the fin is pressed between the boards, the better off you will be.
There are two problems with repaired fins, they are thicker than before, and they are never as strong afterwards. Where this approach works best is at breaks where there are lots of rocks, and hitting them is common. In these situations it's easier to continually make small fixes rather than try replacing fins on a regular basis.