Cabin Deck Coring

[Bud Cary]

Hi

My J40 (#92) has some moisture problem areas in the cabin deck. I have removed the headliner and can easily see some definite areas where the original core is shot (it's black), tapping produces dead sounds and the areas are soft, so to me, the diagnosis is clear.

I intend to try and recore it, starting with a smaller test area, from inside the cabin to preserve the outer deck surface but had two questions to start:

Is there a preferred coring material for the cabin deck? Research I've done seems to point to using Coosa and Nidacore, but assuming I can seal it off the recored area really well, is original balsa core just as good, especially since the cabin top is not flat. Also, does anyone know how thick the coring material is so I could order a little material before I start cutting the test area?

Second question, is I'd welcome any comments and suggestions on how to best approach, what to not screw up, etc

Thanks in advance

Bud Cary

[David Jade]

Hi,

 

I had to do this on our previous boat. I ended up using balsa core as the replacement as I could not find foam at the time (Divinycell is commonly recommended). Coosa is pretty stiff I think and might not flex to the deck shape. I’m by no means an expert on this.

 

The first thing I did was drill pilot holes from underneath to probe for wet and dry areas. The wet core went far beyond any visible areas unfortunately (and also spanned a bulk head). Once I had the wet area marked out, I took a rotary plunge cutter bit and cut out just the underside of the deck from inside, depth set to carefully not also cut the top. Peeled that skin off and dug out the rotten core. Next I feathered the edges of the skin, both the piece I cut out and the ceiling with a small belt sander so I could re-use it as the main skin piece. I also scratched up the surfaces for better epoxy bonding and then wiped everything with acetone. Then I used thickened epoxy to glue both the new core and the old skin back up into place, held and pressed in place from underneath using a piece of door skin (thin, flexible) plywood cut to shape and a release sheet in between. I braced it from underneath using various adjustable poles, etc… to press it into place as much as possible. After that cured, I removed the plywood support and added layers of fiberglass to bond the old skin piece back to the underside, using the proper amount of overlap for bonding – this is why I had to grind the edges to a taper out a few inches.

 

I would have liked to vacuum bag it but I didn’t have the equipment or knowhow and it’s hard to do upside down and I don’t think it would have put the pressure on it that was needed working upside down anyway. It’s a mess upside down either way, but finding a way to compress it all from underneath was the key. That and thickened epoxy to make working upside down as reasonable as possible – still very messy. Just try and have no trapped air or voids best you can.

 

It wasn’t pretty to look at from underneath but it worked well and any ugliness was under the headliner sight-unseen.

 

Hope this helps,

David             _/)

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[jeff thayer]

Bud, I have done the same as David.   Getting a good bond is the key.  Clean everything really well after removing the old core.    Use an appropriate resin (test it beforehand so you know it will bond well).  Any voids (air pockets) will greatly weaken the structure so avoid them.   Vacuum bagging is a great way to both remove trapped air and apply a lot of pressure (1 sq-ft will have over 2,000lbs of force applied).  Can't do that w/ stick braces.  Getting a good seal around the edges of the vacuum bag will also be key for that to work.  Test every step before applying final resin.   You don't want to learn about issues when everything is gooped up.  

Jeff Thayer

SV Allez

J/42

[Bud Cary]

Thanks Dave and Jeff for the insights

 

Jeff,   did you also use balsa for the core material and do either of you remember what the thickness is ?

 

 

 

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[jeff thayer]

Bud, I don't remember the thickness of the balsa I used and it was on the deck of a Santa Cruz 27 so not relevant to J-boats.   

Jeff Thayer

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[Owen Harren]

I've done this effort as well with lots of photos I'm happy to share.  Send me a DM if you like!

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[David Jade]

I don’t remember the thickness but as I tried to measure it I was sure it was thinner than what I could source by almost 1/8”. Tried to use a depth gauge to measure it. So I ended up sanding down the new balsa a bit to fit better. Or course, it was not actually thinner in the end - it was just really hard to measure in place and I was left with more space to fill with fiberglass. 

So my lesson was, if it seems like an oddball thickness it is probably not and pretty standard. 

David                      _/)

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On May 23, 2025, at 10:56 AM, Bud Cary <afca...@gmail.com> wrote:



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[Bud Cary]

OK   … thanks again for your other recommendations

 

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[Bill Bowers]

Hi Bud,

TPI coring is 3/4" balsa.  

I am having great results using Raka 127 low viscosity epoxy with mostly 350 non blush medium sped harder for J42  keel grid repair with heavy biaxial cloth.  If you cannot use vacuum bagging it is difficult to get air out of heavy cloth or balsa with normal resin viscosity so you will have dry laminate air pockets.

Bill Bowers 

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