Finbox repair approach....

I recently bought a Bing swallow tail circa 1970 that needs a finbox replacement.

I routed out the box and cleaned up the opening so it has smooth edges and looks nice.

I’ve searched the archives and have found lots of good info on this topic. My question is about approach.

Should I fill the finbox hole with a resin/qcell mush or should I rap the new box in lots of cloth then fill the hole with resin? Since I routed out the old box, the hole is much wider than the original one.

Any suggestions?


It works well if you

  1. put a fin in and tape off the top of the box

  2. dump a load of Q-cell/lam resin in the hole

  3. squish the box in, so the box sits about 1/8 inch below the deck.

  4. put a straight edge or two across the deck (the resin expands as it

cures, and will otherwise lift the box out. It’s gonna get tricky anyway, the resin really wants to increase its volume

  1. clean it up around the deck after it gels, before it cures.

  2. take 1/16" off the top of the box

  3. tape off the inside of the box.

  4. cut the glass close around the box inside edge

  5. apply lam resin, hot coat, sand, gloss

If you want extra strength, make four small holes at the corners of the box that go through to the top deck, and fill these with the Q-cell/resin first before you put the box in.

Some boxes have lips for glassing over, these are easier. Just glass over the top and grind it down.

I believe at least one layer of glass & cabosil is stronger than just cabosil.

From Moonlight glassing; you can see the box taped off, glass sticking up & cabosil resin all over the place:

Thanks for the replies.

I think I’ll end up using a combined qcell/cloth approach. I want it to be cosmetic as well as functional b/c it’s an older board that I’ll probably only ride a handful of times.

When I do that, I like to first just rout out enough of the old box to get a new box in…for future reference. However…

I have done a few of these and the way I like to do it is to bed cloth ( 10 oz, diamond pattern patch ) in some soupy cabosil mix. Tape your cloth down, make a double Y cut in the cloth in the box area ( show ya what that is in a moment ) , put in a little cabosil mix ( gauge by eye, too much is bad, too little is worse) in the hole, under the cloth, and wiggle in your fin box, with a fin in it and the rest of the slot taped up good. It’s not a bad idea to make a centerline mark front and back on the finbox so you can line it up with the stringer… something I remember to do most of the time.

In a perfect world, you’ll have the cabosil just flush under the cloth, soupy enough that wiggling the box will get out any bubbles. If it’s a little low, that’s fine, 'cos then you put the lam coat to the cloth patch and it soaks in and fills the rest of that hole beside the box. Leave the box flush or a skosh high. Squeegee the resin to the tape, when it’s gone off to a gel stage you can cut it carefully and have a nice clean edge. Sand the box flush if it isn’t already, feather the edges of the patch a little, hotcoat, gloss, go surfing.

The cloth ties the board and the finbox together pretty well, beefs up the area some and hides a few things like router slips that you really don’t need the rest of the world to see. You can do it with a tint or opaque color for the 'I meant to do that ’ look.

hope that’s of use



you totally rule!! thanks.

a silly question: is cabosil the same as Q-cell?

If not, then I’ll need to order some of that.

Thanks again.

Hey, Josh

Cabosil, aerosil, fumed silica, microbaloons, q-cell - hell, I’ve used somebody’s scented talcum ‘body powder’ that was lying around when I had nothing else - they all work fine as thickening agents. Whatever you got is fine. Though the talcum smells a lot nicer when you sand it.

Make a soupy batch, a little more liquid than well-stirred yogurt but definitely thicker than half and half cream, lam resin is fine if you don’t have sanding resin, and I’d go quite light on the catalyst, just in case it takes more futzing time than you expected. Thick castings like this go off a lot faster than you’d expect, so do it out of the sun and ideally in the morning cool.

One thing I forgot to emphasize is to use your fin in the box ( well-taped so no resin gets into the slot ) and an educated eye to make sure the box is in there straight up and down. A small square will help a lot with that. You can also use a straight edge and eye along the stringer to the leading and trailing edges of the fin to make sure it’s in there straight and not aiming towards the rail. Crude but effective. Use masking tape to the fin and the rails to hold it there and recheck now and then while the resin gels. Then you can pull the fin to sand it, retape the box to hotcoat and gloss and there you have it.