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Old 08-02-2016, 06:26   #1
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Replacing wet balsa core

Hi i just thought i'd post few pics and my plan to fix the issue I'm dealing with and I was wondering if anyone has any other suggestions.

Wet balsa core is along the genoa track on C&C38 mk2 from 1976.
area is 12" wide and 14 feet long. Top laminate is 3/16" thick and balsa is 1/2" thick. That 12" width is pretty much the extent of rot as deck tapers at either end.

I plan to use either west or MAS epoxy to do repairs, and get this 24oz double bias stitch mat.
24oz. Double Bias Stitchmat. I'll taper around about 2" additional inches to allow for adhesion.
There was a 4" wide section of marine ply under the track, I am guessing its for loading purposes. I am wondering if i do the same or just put balsa coring everywhere. Originally these boats had a very lousy 1/4" by 1" aluminum backing plate and once water got in, there was no return. A while back i replaced the backing plates with aluminum of 2" wide by 3/8" thick in hopes of improving the situation but I had no idea whats beneath the surface.
I plan to seal new balsa with epoxy and use thickened mixture to fill voids around balsa before laying the glass.
Then at some point, I will have to transition to polyester (gelocat) to match the rest of the deck. Either that or do 2-part nonskid. I worked with interlux before and i thought it was pretty good and durable. I haven't made up my mind on that yet.
Any suggestions appreciated.
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Old 08-02-2016, 06:37   #2
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Re: Replacing wet balsa core

Balsa will rot again, sooner or later.
Why don't you use Airex instead?

Airex - highperformance rigid foam core material¬*- 3A Composites AirexBaltekBanova
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Old 08-02-2016, 07:58   #3
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Re: Replacing wet balsa core

Use Coosa board and stay away from the epoxy. Do the whole job in poly and it will be much faster and cheaper, and just as good. Loose the 24 oz. as well, use 1708.
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Old 08-02-2016, 09:12   #4
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Re: Replacing wet balsa core

Quote:
Originally Posted by minaret View Post
Use Coosa board and stay away from the epoxy. Do the whole job in poly and it will be much faster and cheaper, and just as good. Loose the 24 oz. as well, use 1708.
I am assuming you say stay away from epoxy because of epoxy curing complications and gel coat adhesion. 1708 is very similar to what i listed. 45degree biaxial and chop strand mat. Coosa look really good. So no need for plywood if i use Coosa?
If using polyester, i still add a layer of SCM to bottom skin, then bond coosa with thickened polyster, then fill voids with thickened polyester, then build up 1708 with polyester, then fair with thickened polyester.

I was going to seal balsa with viscous epoxy to prevent future water ingress. But that's why you guys come in. Thanks for the advice.
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Old 08-02-2016, 09:54   #5
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Re: Replacing wet balsa core

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Originally Posted by phorvati View Post
I am assuming you say stay away from epoxy because of epoxy curing complications and gel coat adhesion. 1708 is very similar to what i listed. 45degree biaxial and chop strand mat. Coosa look really good. So no need for plywood if i use Coosa?
If using polyester, i still add a layer of SCM to bottom skin, then bond coosa with thickened polyster, then fill voids with thickened polyester, then build up 1708 with polyester, then fair with thickened polyester.

I was going to seal balsa with viscous epoxy to prevent future water ingress. But that's why you guys come in. Thanks for the advice.

Yes to gel adhesion problems with epoxy. If you use epoxy, you must finish with paint. Unless the boat is already in paint instead of gel, this means painting the whole boat. 1708 much easier to work with. No need for ply with Coosa as long as track is through bolted with backing plate. No CSM on bottom, will throw off core thickness. Bed in CoreBond if you can get it. Make sure to use the BPO catalyst if possible. Fair in Rage. Brush prime with gel.
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Old 08-02-2016, 12:38   #6
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Re: Replacing wet balsa core

The plywood was there as it wouldn't compress under the bolts of the track. A large backing plate will moderate the compression issue with balsa core but you still need to be careful and not over tighten the fasteners. Proper bedding with Life Caulk and a slight countersinking of the fastener hole should stop any future issues with leaks into the core. Routing out the core around the puka with Dremel 199 bit, filling with thickened epoxy and redialing the hole should be a permanent fix.
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Old 09-02-2016, 08:14   #7
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Re: Replacing wet balsa core

Quote:
Originally Posted by minaret View Post
Use Coosa board and stay away from the epoxy. Do the whole job in poly and it will be much faster and cheaper, and just as good. Loose the 24 oz. as well, use 1708.
Here's a link with some technical information regarding polyester gel coat over epoxy.
WEST SYSTEM | Projects | Fiberglass Boat Repair and Restoration - Polyester over epoxy

I'd use a foam core material with plywood or solid glass under the track. If you vacuum bag the layers together you can ensure that all voids will be filled with epoxy.
Epoxy makes stronger secondary bonds than polyester resin does.
You don't need to add glass to the bottom skin unless it's somehow damaged.
In areas where you are mating to a rough surface you could add some high strength filler to thicken the epoxy a bit.
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Old 09-02-2016, 08:28   #8
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Re: Replacing wet balsa core

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Originally Posted by guyrj33 View Post
Here's a link with some technical information regarding polyester gel coat over epoxy.
WEST SYSTEM | Projects | Fiberglass Boat Repair and Restoration - Polyester over epoxy

I'd use a foam core material with plywood or solid glass under the track. If you vacuum bag the layers together you can ensure that all voids will be filled with epoxy.
Epoxy makes stronger secondary bonds than polyester resin does.
You don't need to add glass to the bottom skin unless it's somehow damaged.
In areas where you are mating to a rough surface you could add some high strength filler to thicken the epoxy a bit.



If you did a bit of searching on the forum, you'd note this is a subject I have discussed many, many times here. I am well aware of what WEST has to say on the subject, and have done a great deal of real world experimenting on this. The relevant portion of the link you posted for me is:


"The nine-week exposure may not simulate what could happen after many years, but it does indicate that the gelcoat bond to epoxy laminates is a good bond and should perform well."



My experience is that bond failure usually starts to occur between year one and year two, depending on the environment. Yes, gel will pass adhesion tests done shortly after bonding on epoxy. Just try it two years later. Ever wonder why they didn't do that test?


Occasionally, I have a client with a good enough reason to cause me to apply gel over epoxy. I ALWAYS use a tie coat. 545 works great. However, I don't like doing even this. This is because all paints and primers, even 545, are softer than cured gel. And I don't like applying a hard coating over a softer one. It leads to a higher incidence of chipping and fractures in gel.
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Old 09-02-2016, 09:08   #9
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Re: Replacing wet balsa core

Quote:
Originally Posted by minaret View Post
If you did a bit of searching on the forum, you'd note this is a subject I have discussed many, many times here. I am well aware of what WEST has to say on the subject, and have done a great deal of real world experimenting on this. The relevant portion of the link you posted for me is:


"The nine-week exposure may not simulate what could happen after many years, but it does indicate that the gelcoat bond to epoxy laminates is a good bond and should perform well."



My experience is that bond failure usually starts to occur between year one and year two, depending on the environment. Yes, gel will pass adhesion tests done shortly after bonding on epoxy. Just try it two years later. Ever wonder why they didn't do that test?


Occasionally, I have a client with a good enough reason to cause me to apply gel over epoxy. I ALWAYS use a tie coat. 545 works great. However, I don't like doing even this. This is because all paints and primers, even 545, are softer than cured gel. And I don't like applying a hard coating over a softer one. It leads to a higher incidence of chipping and fractures in gel.
to paraphrase;
1. use a weaker material in a high load area to avoid cosmetic issues in the future.
2. there is only one knowledgeable person on this forum.
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Old 09-02-2016, 09:15   #10
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Re: Replacing wet balsa core

Quote:
Originally Posted by guyrj33 View Post
to paraphrase;
1. use a weaker material in a high load area to avoid cosmetic issues in the future.
2. there is only one knowledgeable person on this forum.



To answer 1. No, use a material that is exactly as strong as the original material and the surrounding material it is tied into, instead of creating a hard spot by making a repair with material twice as stiff as the material it is tied into.

No point in answering two. If you think I'm the only knowledgeable person on this forum, it's obvious you didn't lurk long before starting to post.
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Old 09-02-2016, 09:34   #11
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Re: Replacing wet balsa core

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Originally Posted by minaret View Post
To answer 1. No, use a material that is exactly as strong as the original material and the surrounding material it is tied into, instead of creating a hard spot by making a repair with material twice as stiff as the material it is tied into.

No point in answering two. If you think I'm the only knowledgeable person on this forum, it's obvious you didn't lurk long before starting to post.
1. you can not recreate the original strength of the primary polyester bond in the areas where you tie into the existing structure. GRP with epoxy is not twice as stiff as GRP with Polyester; no hard spot to worry about.

2. I was trying to highlight your huge ego without calling you an A**.
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Old 09-02-2016, 09:42   #12
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Re: Replacing wet balsa core

Sometimes those egos are well deserved. I'm grateful that some folks keep posting even after they're constantly attacked for no apparent reason.
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Old 09-02-2016, 09:55   #13
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Re: Replacing wet balsa core

I've done this repair in several places on our 1979 Dufour 35. Your pictures look very familiar to me

We replaced balsa core with balsa. It won't rot out any time soon (or ever) if you don't let water in. I would stick with plywood under the track, also as was original, for compression loads. That said, don't over-tighten the bolts when re-installing. The backing plates and nuts are handling things not the tightness of the nut/bolt combo.

I laid up with 1708 and West Systems. Filled hollows as you described with thickened epoxy. Scarfed the surrounding deck to tie in the new glass and fair, also as you describe. Trying to keep the same overall glass thickness is important. You don't want flexing between adjacent areas due to different thicknesses.

Where the track bolts through the deck I would over-drill and refill with thickened epoxy. Pretty standard procedure for mounting hardware on a cored deck. I also like to bevel the top of the bolt hole so that the sealant can form a gasket around the bolt. Otherwise most gets squeezed out.

I used Interlux Primekote as a primer over the repair area and then Kiwi Grip over that. We are on year 4 for my earliest repairs and the repair and Kiwi Grip are holding up extremely well.

I think you are on the right track. Keep it simple. Good luck!
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Old 09-02-2016, 10:22   #14
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Re: Replacing wet balsa core

+1 Keep it simple.

The original lasted 40 years - and likely would have gone longer without failure. C&C built a quality boat.

There's nothing wrong with making improvements from the original spec but be careful it doesn't create new problems. Boat repair whack-a-mole is no fun.

The perfect is the enemy of the good
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Old 09-02-2016, 12:19   #15
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Re: Replacing wet balsa core

That test was done with regard to a bottom repair & this is a deck situation.
Could be worse in a widely varing temperature situation.
But with a known incompatibility why bother. If you want to use epoxy just paint over. It will be easier to get a good finish with paint than gelcoat.
When you have done as many repairs as minaret you have the right to come across a little uncompromising. Avoids the DOS principle.

A good many of us will ask & then do as we please anyway with varing results.
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