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Old 18-11-2021, 20:03   #1
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Fixing cabin wall plywood rot (Leaky portlights behind FRP)

Pulling out all the leaky portlights. They were never sealed unfortunately so water has seeped into the wall. What to do?

It's a FRP gelcoated board against the fibreglass cabin top. The end grain of the ply is exposed underneath where the headliner is. It drips down there but I don't know if it would dry. It seems to be a double sided FRP. Not too attached to the outer glass in places.

I've attached pictures of one of the worst so far. I dug out a bit of ply with a chisel but it's hard to go further down. It's not total mush. There was a slight osmosis bubbling on a small part of the wall which sanded down easy enough. The board goes 4-5" down until the headliner.

Originally I was going to fill with thickened epoxy and reinstall but then I worried if I should replace any core. That makes a tough job though.

I thought about taking a multitool and slicing the thin fibreglass layer neatly around the whole cabin but I doubt I'd be able to remove it neatly and reinstall. Slicing sections isn't fun because I very much doubt I could match the white gelcoat. Regelcoating the whole wall would be a big pain too having buy a sprayer, etc.

I'd love some thoughts. How structural is this sandwich? Do I need to replace the core or not? I couldn't find replacement FRP in Australia easily.





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Old 18-11-2021, 20:27   #2
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

Hi first of all you have a glass and resin shop at kawana, all or most of the marine supply shops sell some glass and resin, larger supplies are available overnight from about 30 suppliers in Brisbane or the Gold Coast.

Are you capable of the repairs you are looking at, I think that by your questions you may at least need some guidance their are many people around that can help you and guide you if you need that help walk along th foreshore near the slips, and start asking around, otherwise down at Buddina at lawries marina or the marina at mooloolaba you will find many capable tradesman that can assist in any way you want.

If it were me the boat would not leave for ocean water with any rot at all, imagine a wave going over the portlite held only by a thin layer of glass and paint,it could be a total loss if conditions were to change., keep looking and ask for help if thatís what you think you need, even at the glass shops some there may be able to help you.
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Old 18-11-2021, 21:04   #3
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

Thanks, car less right now so that's close but far away! The 4wd is another project after a runaway.

I'm capable of doing the resin work. The question is more about how to go about it.

I've also considered the portlight strength. I mean I had to bang it out pretty hard without screws and just the old mastic holding it. I'll be reinforcing the screw area either way.
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Old 18-11-2021, 23:36   #4
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

If the inner and outer mounting holes of the light match up so they can be through-bolted, I'd use the thickened epoxy method; the missing wood shown in the pictures doesn't look that bad. The critical thing is to make sure that, wherever epoxy meets substrate, the surfaces being bonded are clean and solid. A coat of neat or even slightly thinned epoxy before adding filler is always a good idea here.

If the holes don't match up and the inner and outer parts are held by screws, it's easiest to replace the missing wood with more of the same. Use whatever tools necessary to remove the rotten portions to solid wood, sand/abrade the inner exposed glass surfaces as best you can, cut new pieces to fit in place, coat everything with neat epoxy, assemble with thickened epoxy and clamp evenly and well (waxed paper or polyethylene are your friends here).

The hull thickness shown in the first and second pictures seems to indicate that the primary purpose of the interior ply is screw-holding; unless there's something else involving structural issues, I wouldn't go to any heroic level to replace the rotten parts.

As long as everything is bonded and sealed, you should be good for another 30 years.
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Old 18-11-2021, 23:44   #5
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

Don't start cutting either the exterior or interior laminates just yet.
First must get an idea how deep the damage is.
It's labor intensive, actually tedious is a better description,, rather like dentistry.
Using some drill bits, small chisel, (a Dremel tool is great,) dig down a bit, removing the bad wood from between the laminates, and ascertain if the wood is starting to look better as you go, don't rush it.
Sounding with a Nylon faced hammer may detect good/bad core, but that is a developed skill.
Once you determine the extent/depth of the damage, then repair options can be considered.
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Old 19-11-2021, 00:51   #6
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

There's the thick outer fibreglass then the interior walls are the gelcoated frp. I'm not so sure if that is structural or not. If not, it would make an easy way to finish a boat interior and add some insulation when building. The same material has been used all over the boat like the base of shelves, tops of cupboards, even the galley benchtop.

Perhaps someone more knowledgeable can advise.

I'll take a drill to the end grain from the bottom of the ply soon. I'm about to make a huge mess sanding the sole and fixing some subfloor from a leaky tank so I'm not worried about the mess then. Hopefully any soft ply only rotted downwards. Epoxying in replacement ply sections would be an easier job than cutting any glass and trying make it look good again.

Someone has messed around with it before and added some rubbish hard filler that hardly bonded and crumbled. I have no idea why they didn't seal the wood around the whole portlight. It seems like it's my life fixing other people's junk work on my boat. I think my favourite is twisting wires together and just wrapping with electrical tape leaving a sticky fire hazard.
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Old 19-11-2021, 01:11   #7
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

It could be that the ply is just a solid compressible spacer where the portlights are.
Quite possibly the surrounding areas have a foam sandwich.
I'd carefully take a large drill, drill into the plywood sideways (not through the glass faces) to verify that.
If it's a spacer, dremel/router/hammer out the rotten ply, let it dry, clean it andpaint with lightly thinned epoxy (&hardener!).

Before this is fully cured fill with fiberglass thickened epoxy.
Add some silica gel into this mix, it reduces run off.

Alternatively glue in some prepared pieces of proper marine ply or prefab fiberglass.

If you are in a hot climate, slow the reaction down by using a slow hardener, possibly cool the epoxy parts down before mixing.

If it's not just a spacer close to the portlights, you may need to cut out the inner skin and replace the rotten ply with marine ply, or built up with a solid fiberglass laminate.
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Old 19-11-2021, 03:15   #8
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

When I did a similar repair and a GRP coated ply sandwich, I dug the rotten stuff out as best I could using a wood chisel of suitable width. It smashes the mush, and you can feel when it hits solid core. Once I got back to solid core I cleaned up everything the best I could, made a template out of coreflute (a.k.a. a discarded real estate for sale sign) which was then used to cut a piece of ply to insert. I then glued/filled it all together with thickened epoxy. I could use bolts in my application to fasten it to the outside bits of FRP for a bit of extra strength, but not really needed and your portlight bolts will do the same thing.


Best source of epoxy resin, imo, is Trojan Fibreglass in Sydney. Use fumed silica as the thickening agent which is cheap as chips and just as good as the more exotic stuff. I prefer the 2:1 mix ratio laminating epoxy as the best all round stuff for versatility as it is easier to mix the right proportions at this ratio without the need to use specialist dispensers (you can use cheap disposable medicine cups purchased in bulk at a chemist shop for measuring mixes).
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Old 19-11-2021, 04:27   #9
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

Yeah, I ordered from Trojan. I have plenty of resin, silica, microballoons and glass.

Screw holes don't line up. It wouldn't screw to both sides of the wall otherwise.

I'll try a router bit to chew the soft wood out. Just need a new 4wd motor and I'll drive to the shop and get one.
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Old 19-11-2021, 14:39   #10
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

When I went to replace old an old port light I discovered the screws weren't grabbing anything solid--the inner and outer screws didn't line up. There was thin wood-grain Formica on the interior wall around the port light. I cut away a square of it around the port light with a multi-tool, found the plywood core had rotted to the consistency of oatmeal, then backed further way from the port light until I found solid plywood cut out a square piece of core, and replaced it with an expoxied piece of new plywood. I didn't bother to replace the Formica in the repaired area, but rather painted the interior cabin walls white. No problems so far.
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Old 19-11-2021, 17:25   #11
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

I guess it's a matter of seeing how much rot I have of the plywood. I was tapping with a chisel and hammer so not oatmeal. I could reglass and paint but that may turn it into a huge job and not match the rest of the boat unless I want to continue painting it all. The gelcoat is matt, but I sanded and polished some and it such a nice smooth shine that it's tough to want to lose that.
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Old 19-11-2021, 17:39   #12
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

Hi , I think that Jimbunyard is close to the best practice .I do the same in hole my windows (14) i a old Perry .The unique difference is when all is restore I applied a cover of fiberglass around the porthole and then applied sika 291 in and around the bronze and the put again all in the hole .In each screw applied resin .At moment is strong and work very well .
good work
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Old 19-11-2021, 21:10   #13
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

Hi. I had to do a similar job recently.
Used a multitool and removed the soft ply.Then cut and glued a new ply section.
I will attach piks. Good luck
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Old 20-11-2021, 07:48   #14
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

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QUOTE=Bowdrie;3522461]Don't start cutting either the exterior or interior laminates just yet.
First must get an idea how deep the damage is.
It's labor intensive, actually tedious is a better description,, rather like dentistry.
Using some drill bits, small chisel, (a Dremel tool is great,) dig down a bit, removing the bad wood from between the laminates, and ascertain if the wood is starting to look better as you go, don't rush it.
Sounding with a Nylon faced hammer may detect good/bad core, but that is a developed skill.
Once you determine the extent/depth of the damage, then repair options can be considered.[/QUOTE]
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Old 20-11-2021, 08:35   #15
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Re: Fixing cabin wall plywood rot (Leaky portlights behind FRP)

Yes. Do not cut the skins if you can avoid it.

Regarding the removal of the rotten plywood, the oscillating tool works great.
Good suggestion above. I liken the oscillating tool to an electric Japanese saw. Superbe tool.

You could even cut a narrow blade for it with an angle grinder. Sometimes a pure blade works well enough.
Blades with teeth can do damage much more easily.

Once the old ply is gone you could make a replacement in two parts and slide them in for glueing.
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