Do you have a board in your garage collecting dust because it’s missing a fin box? Maybe that rocky shallow section at low tide, or pot hole riddled dirt road in Baja unintentionally turned your thruster into a twinnie- rendering it unrideable. Don’t worry, with only the slightest bit of DIY gumption and some ding repair techniques, you can resurrect your blade to a rideable state to have back in your quiver for the next swell. In the video above, Jojo Roper walks you through step-by-step how to repair that busted fin box.

When not chasing XXL swells around the world, Jojo Roper is restoring boards to a rideable state at the San Diego surfing institution founded by his father- Joe Roper’s Ding Repair. According to Jojo, about half of all the repairs that come through the shop have to do with fin boxes. Above he imparts the years of experience he’s gained in fixing them. Specifically in the video above, Jojo demonstrates how to repair a Futures fins box.


Angle Grinder

Fin box (either salvaged or brand new)

Sandpaper (40 grit, 150 grit and 320 grit)

Sanding block

4 oz. fiberglass cloth

6 oz. fiberglass cloth




Q-cell filler (cabosil or arosil)

Surfacing agent (sanding resin)

Tongue depressor

Disposable cup

2″ Brush




Masking tape


Step 1 – Preparation

With 40 grit sandpaper, sand the area of the board surrounding where the fin box once was. This creates a scuffed surface for the resin and patch to bond to the board. Open up the delaminated  areas by sanding away the fiberglass that has become separated from the foam. Double check your prep area by making sure there’re no shiny spots, because that’s where the fiberglass patch won’t stick to the board.

Step 2 – Lamination

Installing the Box

Mix some filler by adding Cabosil or Arosil powder (these powders are the exact same thing but have different names due to their manufacturers) into some resin in a disposable cup with a little bit of catalyst until the compound is at a paste consistency.

Then work the paste into the foam for a good bond. Jojo uses a tongue depressor to press the filler into the area where the replacement fin box will go and to level the filler with the bottom of the board.

Take either the salvaged fin box, or a brand new fin box, and insert it into the area you just added the filler to. If you’re repairing the center fin, then make sure the fin is in line with the stringer and straight up and down. If you’re repairing a side fin, measure the correct placement for the box from the stringer to make sure the fin’s tow is correct. Be sure the cant of the fin matches the shaper’s specifications.

Laminating Around the Box

Once the box is set and the filler resin is hard, place a strip of masking tape over the part of the fin box where you insert the fin. This will prevent resin from blocking where your fin goes. Also tape around the rails of the board where your patch that holds the fin box in place will lap over.

Fill in the cavity around the box with more filler resin. Make sure the filler is level with the bottom of the board by using a tongue depressor.

Remove any excess filler resin on the bottom of the board with a rag and some acetone.

Cut a patch out of 6 oz. cloth that covers the whole fin box and that’s slightly larger than the filler resin area and lay it on the board.

Then cut a patch of 4 oz. cloth so that it that covers the whole area that you prepped in Step One. Lay it on top of of the whole repair area.

Pour resin over the patch and use a squeegee to spread it evenly across the repair area and to make sure the patch is flat with no air bubbles. By dabbing the fin box area with a brush that’s been dipped in resin, you can get rid of the little bubbles around the box.

Brush a layer of sanding resin over the whole repair area, this is also known as the hot coat.

When the resin is in a gel-like state, use a new razorblade to clean up the repair. Lift up the masking tape that’s near the rails and cut the gel-like fiberglass as you lift the tape. Most importantly, timing this step right can save time when sanding the ding later.

Step 3 – Sanding

An angle grinder is basically a magic wand for this step. Apply some 40 grit sandpaper to the angle grinder. If you installed a new fin box, there will be a higher bulge than if you reused the old one, sand this so that it’s level with the bottom of the board.

Apply 150 grit sandpaper to the angle grinder to smooth out all of the bumps and lumps that are in the patched area.

With 320 grit sandpaper, sand the repair area with your hands to fine-tune the patch so that it’s flush with the original bottom contours of the board. Also use your hand to sand the rails of the board to make sure it’s shaped properly. Once everything’s nice and smooth, finally it’s ready to ride.

For a “How to” on fixing a buckled board, click here.



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