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Old 10-05-2016, 15:03   #1
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Rotten balsa core - major repair

O'hoy sailors,

So here I am in the north of France with my Sweden Yachts 34 at last getting her over due teak removal, core replacement and synthetic decking installed.

I have cut open all the areas where the balsa was rotten, and am now in the process of planning out the details of the job. I know that is the wrong order, but hey, you live and you learn.

After researching and going back and forth epoxy vs polyester I think I've finally landed on polyester, mainly for cost and less chance of messing up. I might add that I do not have much experience working with either.

The total area I will be replacing is roughly 5 m2, i.e. a pretty huge job...

My main concern with using polyester is that I've already messed up when I cut the laminate and left too little space for grinding the recommended 1:12 bevel. This is true for maybe a total of 4 m towards the cabin deck. The width of these edges varies from as little as 10 mm to 30 mm. The skin itself is 5 mm, so I should have left 60 mm plus. I guess this problem would have been easier to mitigate using epoxy to accomplish a stronger bond.

Any ideas how to get a good enough bond between old skin and new?

I intend to use Divinycell 80 as core material and lay up new fiberglass on top. Maybe I could build up with solid fiber glass and no core towards the edge?

I've attached a couple of pictures. The aluminum toe rail is taken off btw.

I'd love some input on this, it's really bugging me that I didn't know better when I started cutting. Amazing how carried away you get with an angle grinder in your hands.
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Old 10-05-2016, 15:33   #2
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Re: Rotten balsa core - major repair

Although it seems overwhelming, you'll come out with a strong new structure. I think the worst is over...
Because of the area without as much "edge"to taper, up forward (I believe), I would go with SLOW cure epoxy and use biaxial cloth. Do your best to get as much taper as possible. The epoxy ( Mayonnaise- peanut butter consistency) will provide a stronger bond.
Place core material in thickened epoxy -wet out core bottom and lower layer of glass with epoxy, notch trowel thickened epoxy and set core into it. I would glass over the core at the same time. You could section off into six foot units it's less work to do each section completely (wet out bottom layer and underside of core, putty with thickened epoxy, put core in place, lay out biaxial cloth, wet out each layer and bubble roll well to remove all air). I think you'll use less epoxy then you think... to curb the costs.
It's not rocket science but you do need to have a good plan and follow your plan to work efficiently. Don't do it in the sun... You'll be sailing in no time with a strong hull and deck structure.
Best of luck - work that plan
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Old 10-05-2016, 15:34   #3
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Re: Rotten balsa core - major repair

Norado,
As someone who has done just what you are looking at, I used epoxy for 3 reasons:
  1. I am not very good at controlling the pot life of polyester.
  2. I like that with epoxy you have slow, medium and fast hardeners to work with
  3. Epoxy bonds MUCH better to old polyester than polyester does and you want a strong bond.
A couple of suggestions:
  • be merciless with removing questionable balsa
  • mix some 1/4" glass fiber with the epoxy or polyester when you bond the Divinycell 80 core to take up the space. A little fumed silica (Cab-O-Sil) will thicken it up, suggest you get to the consistency of ketchup
  • have some flat boards and weights to hold the Divinycell down when you do it.
  • have some plastic sheeting to cover your work to keep the dew off it overnight
You have already taken the first hard step, cutting it open. Good luck.
Ed
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Old 10-05-2016, 15:38   #4
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Re: Rotten balsa core - major repair

Quote:
Originally Posted by Sailorman Ed View Post
Norado,
As someone who has done just what you are looking at, I used epoxy for 3 reasons:
  1. I am not very good at controlling the pot life of polyester.
  2. I like that with epoxy you have slow, medium and fast hardeners to work with
  3. Epoxy bonds MUCH better to old polyester than polyester does and you want a strong bond.
A couple of suggestions:
  • be merciless with removing questionable balsa
  • mix some 1/4" glass fiber with the epoxy or polyester when you bond the Divinycell 80 core to take up the space. A little fumed silica (Cab-O-Sil) will thicken it up, suggest you get to the consistency of ketchup
  • have some flat boards and weights to hold the Divinycell down when you do it.
  • have some plastic sheeting to cover your work to keep the dew off it overnight
You have already taken the first hard step, cutting it open. Good luck.
Ed

All good suggestions - agree


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Old 10-05-2016, 18:03   #5
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Re: Rotten balsa core - major repair

You want to avoid any flexibility at the edges. Or else the new top may crack up and let water in.

This is often some challenge when the core is foam.

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Old 11-05-2016, 08:46   #6
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Re: Rotten balsa core - major repair

Interesting point on the divinycel flexing at the joint line. Haven't opened up my decks near weeping toerail bolts but may face a similar concern. Toerail bolts also hold hull and deck together, so I'm not sure if I actually have wet core or just wicking along some bolts residing in solid fiberglass.
Assuming the worst, and I'm faced with toerail and wet core adjacent, would a plywood edging strip, say 3" wide be suitable to support the new deck joint ? Then balsa core leading towards centre line of boat. Tx
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Old 11-05-2016, 08:56   #7
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Re: Rotten balsa core - major repair

Don't be afraid of epoxy, it's not hard to work with. Just measure carefully, mix thoroughly & keep the batches relatively small as big batches can get very hot. For anecdotal evidence my neighbor redecked his fishing boat with polyester resin & mat over fresh plywood. Within a year it had delaminated. Many years ago during a haul out I made friends with the shrimper next to me. He was rebuilding his boat & glass the plywood exterior with polyester resin. He ended up selling the boat but told me that the glass started delaminating a few years later & the whole job had to be redone.
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Old 11-05-2016, 09:03   #8
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Re: Rotten balsa core - major repair

Epoxy, epoxy, epoxy. Forget about all the negatives you hear about it.... like the UV problems, etc. You will be much happier, it will be easier to get a good bond, and it will hold well for a longer time. BTW, the advice about small batches is REALLY good. If you mix up a big pot of the stuff, thinking that you can get it all done, you will have a hot mess on your hands. The stuff will begin setting up and the consistency will change from the beginning to the end of the job. You will also tend to rush the job... so take it slow and easy, small batches, remembering that anything that isn't right can be cut out, ground down, or otherwise fixed. Haste truly does make waste in this case.
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Old 11-05-2016, 14:30   #9
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Re: Rotten balsa core - major repair

I think you just scared me back into Camp Epoxy!

To be honest, I believe the job can be done with either or. The more you google the more insecure you get. If I had a lot of experience with both it would be easier to take a stand maybe. There are people saying deck recoring is a 100% poly job and those saying it's a 100% epoxy job.. The guys on the shipyard recommended polyester as it would flex and not generate any hard stress points. I was thinking maybe putting polyester on the bottom skin with one cloth and then finishing with epoxy on top. I'll get a quote for 20 liter cans of West epoxy tomorrow, it'll be exciting to see what the party's gonna cost me. If epoxy was the same price as poly it would've made it a lot easier. Of course, redoing the job will be vastly more expensive than the price difference epoxy/poly.

Thanks for your advice and support Chenega and Ed, it's much appreciated.

Today I've been digging out old balsa from where I did leave some skin for tapering. It's really time consuming getting it all out. Is it gonna be a problem if there is just a little balsa left up in the corner, as long as it's dry? I don't mean chunks, but It's a real pain getting every grain of wood out of there. I'll sand underneath with a disc sander but it wont get all the way in there.

Thanks again everyone.
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Old 11-05-2016, 14:35   #10
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Re: Rotten balsa core - major repair

Quote:
Originally Posted by Chenega View Post
I would glass over the core at the same time. You could section off into six foot units it's less work to do each section completely (wet out bottom layer and underside of core, putty with thickened epoxy, put core in place, lay out biaxial cloth, wet out each layer and bubble roll well to remove all air).
Why would you choose this approach instead of laying all the core and then glass over at a later time (in sections)?
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Old 11-05-2016, 14:53   #11
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Re: Rotten balsa core - major repair

Yes, this is a messy, stinky, miserable job. When you start it, you think you are going to ruin your boat. You tear up perfectly good fiberglass to get to....yukky stuff.

That is why some people take their drills, drill a zillion holes in the deck, let it dry out, pump denatured alcohol in there (drying agent), let it dry, and then pump it full of liquid epoxy and let it set up. This is a less messy approach, but honestly, only good for small sections, and places where there is some good, some bad. My problem with it is you never know that you got it all....or that the bond is OK. Better to rip it off, do it right, and then you know. After all, your life could depend upon how good a job you do... especially if you're offshore or caught in a squall.

If you want to leave the good stuff in there, no problem, but it makes the job a lot more difficult. You will have a patchwork of oldy goody and new. My suggestion is to minimize that. Around the edges maybe... but really, just pull the junk out. Use a chisel.
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Old 11-05-2016, 16:07   #12
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Re: Rotten balsa core - major repair

There are plenty of other examples of folks doing recoring jobs, right here on this site. Plus, such a job is usually pretty well covered in the technical references provided by the companies which make & sell; epoxy, cores, & reinfocements.
And you're already off to a good start, in addition to asking the right kinds of questions. So you'll do fine.

Regarding the overhyped fears of epoxy pot life. Go with a slow curing mixture, & a wide, shallow mixing pot, is far better for this, than a narrower, deeper one. As the depth of epoxy (that you mix up), generally, is the big factor in controlling how quickly it kicks off. So shallower is better.

Also, there are numerous ways to eliminate any concerns about core flex (cracking). Though foam is pretty flexible to begin with. Plus, with the proper thickness of glass atop it, a deck really won't be doing any flexing of significance.
The only way that that might be a concern, is if the core & deck, were of marginal thickness (or less) to begin with. Otherwise, you're good.
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Old 11-05-2016, 17:09   #13
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Re: Rotten balsa core - major repair

I've been using resin and cloth/mat since the 1960s. Poly only bonds when the previous layer isn't fully cured. I don't care how you sand or wipe down, it doesn't bond like epoxy. I had a marine business that built and repaired commercial vessels. Other than fiberglass hull repair, most of the glass work was decks and cabin tops for wood boats.
Previous poly decks could be removed with little effort. Sometimes in a single pull. Epoxy with some high density filler put down before the 1st cloth layer will bond better than epoxy alone. Epoxy will bond to almost anything without excessive methods. Epoxy decks I did lasted 20+ years on decks that were dragging pots and gear across.
My current boat is wood and I redid the fiberglass decks 5 years ago with West Systems. Considering how much a wood hull flexes, not a crack or delam.
If your going to leave epoxy in the sun (UV) for an extended time, add some HD filler in the top coat. It seems to block the UV.
I don't know int'l shipping costs, but Discount Marine Supplies offers quality boating supplies & marine supplies for boaters and sailboat owners. has the best West prices I have found. $85/gallon for resin. Probably better than the local marine store.
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Old 11-05-2016, 17:14   #14
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Re: Rotten balsa core - major repair

Quote:
Originally Posted by Dymaxion View Post
Interesting point on the divinycel flexing at the joint line. Haven't opened up my decks near weeping toerail bolts but may face a similar concern. Toerail bolts also hold hull and deck together, so I'm not sure if I actually have wet core or just wicking along some bolts residing in solid fiberglass.
Assuming the worst, and I'm faced with toerail and wet core adjacent, would a plywood edging strip, say 3" wide be suitable to support the new deck joint ? Then balsa core leading towards centre line of boat. Tx
I may be wrong but these bolts should not go thru the core.

Cores are normally terminated say an inch off the edge and then all edge fittings pass thru just solid grp.

At least this is how they did it back in '67 where our boat is from (Sweden).

I believe a solid piece of epoxy saturated plywood will be as stable as solid grp, should you at all find yourself in the position where the edge needs extra support. However, you can cut out from below and avoid all the worries altogether.

b.
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Old 11-05-2016, 17:26   #15
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Re: Rotten balsa core - major repair

Wow !

First thread I have ever seen with nothing but excellent advice.

Stop right now and don't read another thing posted on this thread.
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