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Old 19-03-2013, 21:10   #1
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Repair on balsa core rot in deck

I just came from another thread where Minaret told us that every boat with balsa core has rot. He seems a bit disgusted with balsa cored boats but then again his current boat doesn't have balsa core and on CF we always claim that what we don't have is bad, bad, bad

But he might well be right because my high tech SCRIMP process layup with vinylester resin did develop a soft spot on deck which I was sure was core rot.

This case was rather easy to deal with because it is a very small spot and there is good access from below. I'm also not impressed much by fiberglass damage because I worked with it since building my first (Mirror) dinghy.

First step was to take a drill with 4.5mm bit outside and drill holes all the way through around the perimeter of the soft spot. There were small but clear circular cracks around two areas, each baseball size and I drilled in the center of each too., clearly this is impact damage and water got through these cracks into the core. Here is the first picture taken from underneath and after this drilling:

The discoloration already suggests that the damage goes a bit beyond the outline of the soft spot as can be expected. We have a 1" thick balsa core here.

I decided to tackle this from underneath in order to limit the repair work on deck as much as possible. This makes it a bit tougher to do because gravity works against us, but I got used to that anyway
I got a 60mm holesaw from my collection and opened it up:

This immediately removed the worst of it plus it allowed to remove more around these holes. I later had to make two extra holes for a total of 4 in order to get to everything. After removing all soaked balsa it's time to dry everything out, for which I use a continuous fan and a heatgun every hour or so to blow it out with good hot air. Using a small flashlight You can easily see in between the fiberglass layers if there is more rot or not.

I did find that the pieces of end-grain balsa were encapsulated by resin, but not so well as I thought it would be. It almost looks like the rot was able to pass through into the next balsa piece, dissolving these barriers. From now on I will state that water damage will be limited to the damaged balsa pieces plus one circle of pieces around those for every 10 years it has been wet. The pictures still show very clearly a hard line for the extend of the damage and those are the resin-filled spaces around the pieces of balsa. Only with the SCRIMP process you get the encapsulation to this level so that damage is contained.

The next step is the hardest part and that is to make the upper skin water tight again. There are too many cracks to fix them from above, so I will laminate a single layer of woven cloth from beneath , through these holes, to the underside of the upper layer of fiberglass. This is where gravity will be my enemy but it'll be okay. I have 12 ounce woven cloth and epoxy, both from West System, that I'll use for this. When the upper skin is waterproof again, we'll have to fix up the core and then close the bottom. I will then likely re-drill some holes from the top and the bottom to use for injecting a slightly thickened resin to fill up the voids that I will have in the core. The deck is slanted pretty good here so I can do half or more of the injecting from below. I will still drill some holes from the top for that so that air can escape and I can see resin come out when the area lower is filled up. I'll add photo's.

I was very close to grabbing the Fein Multimaster and cut out a rectangle of the visible spot instead of just these holes. It looks like it will be easier and faster that way and that is surely true for fixing the upper layer of glass. But from there on it will be more difficult for putting on a new core and new lower fiberglass laminate restoring original strength or better.

Another option is to do it from above and this is the quickest way until it's time to do the finishing. These skins are very thin and I like to keep them intact as much as possible.

To be continued...
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Old 19-03-2013, 22:05   #2
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Re: Repair on balsa core rot in deck

personally i would be inclined to do it from above!
pouring in thickened resin and filler will fill any voids much more effectively than trying to push it up hill!.
then reglass,fair and recoat the deck
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Old 19-03-2013, 22:21   #3
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Re: Repair on balsa core rot in deck

Balsa core has gotten a bad name because of shitty jobs installing hardware. My 44 year old boat has no issues with rot in the balsa core. Probably not because abnormal care in installing hardware but because there was very little hardware installed where there was core. Such notorious leak areas as lifeline stanchions were installed into solid glass so the leaks there didn't get into the core.

Balsa core actually retards rot as it is vertical grain. Any water intrusion has to work it's way through each cell of the Balsa to get to the adjoining one. Drastically slows down the advance of the rot. Plywood, on the other hand, wicks the water through the wood and tends to rot out in large areas. Foam doesn't rot, but it can delaminate from the FRP which makes it act essentially like rotten core.

Have never tried a repair from underneath, gravity is such a wonderful discovery.
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Old 20-03-2013, 04:55   #4
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Filling up the voids by injecting resin will be done from above using gravity to help; but making the top layer fiberglass watertight again and restoring the core will be done from below. The deck is not level, which can't be seen on the photo's, but this means that injecting partly from belowdecks is still injecting from above and using gravity.

It's all dry now so will continue today.
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Old 20-03-2013, 05:03   #5
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Re: Repair on balsa core rot in deck

Quote:
Originally Posted by s/v Jedi View Post
Filling up the voids by injecting resin will be done from above using gravity to help; but making the top layer fiberglass watertight again and restoring the core will be done from below. The deck is not level, which can't be seen on the photo's, but this means that injecting partly from belowdecks is still injecting from above and using gravity.

It's all dry now so will continue today.
i'm sure you will do a great job!

having done lots of gelcoat repair in the past getting the outside back to its former glory for me is not a big deal!
plus working outside in the breeze and making the minimum dust inside are my considerations working in the tropics.
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Old 20-03-2013, 07:47   #6
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Re: Repair on balsa core rot in deck

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plus working outside in the breeze and making the minimum dust inside are my considerations working in the tropics.
I would almost start over

Actually, I have done a similar job from the outside when I needed to replace (good) core with solid laminate for aft deck cleats. But it didn't need any gelcoat because it was covered by the cleats. I never get the color right so I don't dare to do it in plain sight


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Old 20-03-2013, 08:30   #7
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Re: Repair on balsa core rot in deck

Everything you need to know.......

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Old 20-03-2013, 10:15   #8
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Re: Repair on balsa core rot in deck

Just an idea, but I like the concept of "what you can't hide make a feature of!"........

......in this case what about finishing off the holes with round stainless steel blanks? (flush to the deck) - if only to give the next owner countless hours of fun trying to work out WTF they are for!
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Old 20-03-2013, 10:37   #9
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Re: Repair on balsa core rot in deck

[QUOTE=s/v Jedi;1190100]I would almost start over

Actually, I have done a similar job from the outside when I needed to replace (good) core with solid laminate for aft deck cleats. But it didn't need any gelcoat because it was covered by the cleats. I never get the color right so I don't dare to do it in plain sight

i get where you are coming from!
just make sure the leaks that caused the problem in the first place get a reasonable overlap when you glass the underside.

a little trick when upside down glassing is to cut your cloth to size,lay it on a piece of oven paper and wet it out.

wet out the surface to be glassed.

roll up your wetted out cloth in the oven paper then take it to the repair and with a roller start glassing the cloth to the underside un rolling as you go.
once it is up there,peel off the paper,and repeat with more layers.

if using epoxy thickening the mix slight ly with collodial silica helps as well.

saves lots of drips and mess this way
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Old 20-03-2013, 11:09   #10
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Re: Repair on balsa core rot in deck

Atoll and I have the same teacher... using oven paper right now

I think I'm done for the day because I want a full cure before continuing.

I first cut pieces of 12oz woven cloth and after wetting out all the voids, worked the strips through the holes up onto the underside of the top layer of fiberglass, covering everything and overlapping plenty. This is much easier than it sounds. I then broke one of my own rules and used matt to fill up the voids around the holes where the core was taken out yesterday. Epoxy will immediately break down the cellulose used to keep matt together making one big mess, but I just wet out smallish pieces on oven paper, then rolled them up into sausages (without the paper of-course) like someone else suggested in the balsa core thread and worked them into the voids around the holes. This was much easier than I had hoped because I got it rolled up before it falls apart with time to spare. I'm now left with the holes plus about an inch around them, and will use that to lock in the plugs that will fill up the rest.

It's too messy to take pictures right now but I got a couple from before the epoxy got mixed:

All the bad core removed and everything cleaned up, ready for rebuilding:


All we need for today (except the fiberglass matt) ready so put away the camera and start mixing epoxy
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Old 20-03-2013, 11:22   #11
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Re: Repair on balsa core rot in deck

Just curious - why are the West tins taped up?
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Old 20-03-2013, 11:26   #12
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Re: Repair on balsa core rot in deck

obviously not many tricks left that i can teach you!

apart from one i learnt whilst glassing the hull of an arab dhow on the beach in zanzibar.

first train your workers, then find a nice tree to sit under and a stick for pointing,open a bottle of tusker beer and watch the workers get really sticky whilst muttering words of encouragement
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Old 20-03-2013, 11:43   #13
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Re: Repair on balsa core rot in deck

It stopped dripping so here's the messy part; I promise it'll look better after some cutting and grinding and sanding


Here we wet-out a single layer of 9oz woven cloth (4" wide tape actually) to cut wafers from later, for the finishing of this:
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Old 20-03-2013, 11:48   #14
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Re: Repair on balsa core rot in deck

Quote:
Originally Posted by David_Old_Jersey View Post
Just curious - why are the West tins taped up?
I like to use them for a long time and just add resin and hardener from new cans into these old ones. Has to do with being Dutch and trying to use even the last drop

Quote:
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obviously not many tricks left that i can teach you!

apart from one i learnt whilst glassing the hull of an arab dhow on the beach in zanzibar.

first train your workers, then find a nice tree to sit under and a stick for pointing,open a bottle of tusker beer and watch the workers get really sticky whilst muttering words of encouragement
I am so way ahead of you I even got the T-shirt, check it out here:
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Old 20-03-2013, 12:15   #15
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Re: Repair on balsa core rot in deck

Quote:
Originally Posted by s/v Jedi View Post
I like to use them for a long time and just add resin and hardener from new cans into these old ones. Has to do with being Dutch and trying to use even the last drop



I am so way ahead of you I even got the T-shirt, check it out here:
the real trick is getting your workers to fit in tight spaces,once i had shown the guys how to do it they were great

emergency repair in the sudan on a ferro boat i had
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