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Old 26-08-2010, 09:54   #1
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Wet Core Help

I have been removing all hardware on the deck of my 1975 Downeaster 38 as part of a total refit. When I remove a piece of hardware I have been preparing the holes to be sealed as shown here:

http://www.pbase.com/mainecruising/sealing_the_deck

In quite a few of the holes that the handrails were bolted through I found some wet core when removing the core between the skins for potting. To investigate, I cut out a piece of the inner skin to see what the core was like around one of the more wet holes and it looks like this:



Zoomed out for size reference:



The core material is some sort of dense foam, not balsa. The dry core I've found elsewhere on the deck is a much lighter color, the core in the picture is wet and darker. The core itself is solid and not rotted away, its just damp. There is no delamination on the top or bottom that I have found.

I am trying to figure out what steps I should take now. Since the foam is solid but damp, do I leave it alone and just pot/epoxy the holes? I fear that the wet core might be bad for the epoxy to cure in the holes or even worse for structural integrity. Should I cut out the inner skin wherever the core is wet so it can dry out, then I guess patch it up with a few layers of new glass?

Any advice would be helpful. If the core was rotting away I would not hesitate to just replace it, but since the core is solid but just damp I'm not sure how to proceed.
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Old 26-08-2010, 09:58   #2
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Mine was over balsa, but basically you just gut anything that's rotten, make sure the whole thing is bone dry, and lay in some new epoxy. I'm not sure how big that hole is in yours but a cloth mat designed to be impregnated with epoxy is probably a good idea for structural reasons.

Rebel Heart - Sailing, cruising, liveaboard blog and website - Eric's Blog - rot repair with*epoxy
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Old 26-08-2010, 10:27   #3
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Epoxy (at least that not designed for wet surfaces) will cure improperly if there is moisture present. IF you have no ignition hazards, and can saturate the core with denatured alcohol, (be careful using solvents like lacquer thinner with urethane foams), it will mix with water and raise the volatility of the solution. This will make it evaporate faster. Good ventilation will also speed drying. An alternative for interior areas would be to seal the boat and run a dehumidifier inside for several days.

A more labor intensive (but overall quicker) solution would be to remove the wet core and replace with dry.

Either way, When Completely Dry lay up glass and resin to the original thickness.
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Old 26-08-2010, 10:59   #4
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You are lucky that the wet core is synthetic and won't rot like balsa. You are also lucky that you can get to it from below and you don't have to worry about non skid surfaces.

I would cut out from below the skin over the wet core and replace it. Then fiberglass over the new core.

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Old 26-08-2010, 11:13   #5
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Take out the wet core and redo it with balsa - a layer of matt and then a few layers of biaxal. Don't bother with fancy and expensive core alternatives - after all, you've got a problem right now and it has nothing to do with rotted core, does it? It's wet core - the same problem you'll have again unless you rebed everything properly this time (as the builder did not).

Synthetic cores such as Divinicell cost 3x as much. Balsa is just fine if done right.

For obvious reasons, the only practical way to do this (I am in the process of doing it now, btw) is top down.
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Old 26-08-2010, 12:24   #6
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I guess I should say that the core is damp, not "wet". Either way, what are the consequences of leaving it damp and just sealing it up? I know a problem would be if the water in the core froze, then some delamination might occur. But this boat is in Florida and headed to the tropics once the refit is complete.

What is the major issue if I leave the core damp?
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Old 26-08-2010, 12:37   #7
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Simply damp may not be an immediate problem, but water is getting in and it won't get better and is likely to get worse. Cracked deck at stanchion joints, leaking hardware or toerail are often the culprits (if you don't have teak decks). So that ought to be fixed.

Could you get away with doing nothing? Maybe. It's hard to tell without seeing it.
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Old 26-08-2010, 12:50   #8
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There was somebody on here a few months back who had balsa core and he cut the bottom out of a microwave oven and drilled holes in the top fibreglass and then microwaved the balsa dry. His reasoning was that fibreglass is microwave transparent.

Since microwaves work by vibrating the bond in WATER molecules, perhaps this method would work for you?

I'm sure GordMay knows exactly where that post is......

Edit: Found it using the custom search on one of GordMay's post

Using Microwaves to Dry a Wet Deck Core
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Old 26-08-2010, 12:55   #9
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By the way, I forget the term but essentially salt will always cause things to remain damp. It absorbs moisture from the air, and even if you dry it out, it will go back to absorbing any moisture from the air again. It's one reason the interior of boats are always so damn musty and humid. People who can hose off the interior of their bulkheads and pump it all overboard have bone dry boats that smell great. So if any of the stuff in that foam is salty, it will *never* dry out completely and will always go back to trying to retain moisture. Just cut it out and jam some new stuff in. You could use a chunk of a boogie board; just get the old crap out.
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Old 26-08-2010, 13:01   #10
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It would be best to dry out the core. Since it's foam, not biodegradable Balsa, being wet is not life threatening. If you have the time, close the boat up and run a heater inside to raise the deck temp. Pulling all the hardware off will let the water vapor exit via the fastener holes. May take quite a while, months, for all the moisture to depart. Removing the core around all the fasteners and filling with thickened epoxy, as you are doing, should negate any further water ingress.

Unless you belief in self flagellation, I wouldn't cut out and replace the core. It's not going to gain you much except a whole bunch of work. Anyone who has a cored deck should fill the pukas with epoxy, btw. Oh, and don't use sillycone as a bedding compound. That's guaranteed to leak.
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Old 26-08-2010, 13:15   #11
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Damp core will not do the job it is supposed to do, and that is reinforce the fiberglass layers. It will be soft and you will end up with spider cracks on the deck. That core was installed very poorly. It should be bedded in with a compound made especially for that. If done properly water wont migrate down the channels, as they will all be filled.

The areas under hardware should be solid laminate or very high density core material to keep the outer laminates from flexing when they are stressed.

I would recommend you get Dave Gerr's book, "The Elements of Boat Strength" and read the section on "Fiberglass Sandwich or Cored Construction". There is other good information around about using different types of core materials as well, such as the West System web site.
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Old 26-08-2010, 13:18   #12
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You say there is no delamination, however the bottom skin has come off with no damage to the core. The core should be bonded solidly to the skin and removing skin would require destruction of the core. I can't tell from the picture how the top skin is bonded, but it should be a complete and solid bond. As I noted in the previous post, all of those channels should be filled with a core bonding material.

http://www.powerandsailmarinesurveyo...balsa_core.htm

http://www.westsystem.com/ss/assets/...lacingcore.pdf
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Old 26-08-2010, 14:16   #13
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Quote:
Originally Posted by DeepFrz View Post
You say there is no delamination, however the bottom skin has come off with no damage to the core. The core should be bonded solidly to the skin and removing skin would require destruction of the core. I can't tell from the picture how the top skin is bonded, but it should be a complete and solid bond. As I noted in the previous post, all of those channels should be filled with a core bonding material.

Bonding End Grain Balsa Core with Epoxy Resin Systems

http://www.westsystem.com/ss/assets/...lacingcore.pdf
The skin was bonded pretty good to the core when I cut it out. I had to use a chisel and hammer to peel it off.
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Old 26-08-2010, 14:46   #14
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The skin was bonded pretty good to the core when I cut it out. I had to use a chisel and hammer to peel it off.
Oh, ok. My bad. From the picture it looked like maybe it hadn't really bonded at all.
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Old 26-08-2010, 15:05   #15
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If you have access to back issues of "Passagemaker" magazine, Steve D'Antonio had a couple of good articles on core construction that may be helpful.

Here is a link to a short, concise paper on using end grain balsa core.
http://www.corematerials.3acomposite...n_Part_III.pdf
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