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Old 08-09-2015, 15:41   #1
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Lightweight but strong backing material

Guys,

We have two portholes that have leaked around the cutout installation hole and have allowed water to weep in.

The result is some rot.

The "layup" of the coach side from outside > In is:

1/4" or so Fiberglass
1/4" Backing material (it may be plywood or it may be balsa or teak I do not know)
1/4" of decorative wall teak panels (this is what you see in the cabin).

The problem is that I need to replace the backing material. I wonder if there is a better material than marine ply. One that won't rot or get mildewy/moldy. It would have to be as light as ply, as crush proof as ply, and also hold a screw well.

Starboard seems too heavy and is also too brightly colored... I would be concerned that the white would show through the small cracks between the teak panels.

Any ideas? What do the use in modern boats?
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Old 08-09-2015, 15:47   #2
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Re: Lightweight but strong backing material

McMaster-Carr

There are a lot of alternative . . . But that will most definitely work.
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Old 08-09-2015, 21:11   #3
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Re: Lightweight but strong backing material

G10/FR4 is my go to for this as well.
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Old 09-09-2015, 20:00   #4
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Re: Lightweight but strong backing material

I assume you have fixed the leak...
I would still go with ply - marine ply that is. Just give it a couple of coats of pure epoxy all around and it will do the job. To use G10 or the like would be overkill for this purpose.
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Old 12-09-2015, 15:12   #5
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Re: Lightweight but strong backing material

I happened to run into a guy today at Sailor's Exchange who overheard me asking about where to get a local supply of marine plywood.

He said that any BC quality plywood they sell at Home Depot is the same marine plywood. So, I went and checked it out.

The Georgia Pacific wood they sell as "BC sanded plywood" has the following classification on their website: "Exterior – Plywood suitable for repeated wetting and redrying or long-term exposure to weather and other conditions of similar severity."

I am going to follow the recommendation of a previous poster and seal the wood with west system epoxy by warming it to suck in the epoxy well. I will cut it about 2" larger than needed and create an epoxy seal.

Now, the difficult part will be matching the interior teak when I sand and clean it. Cabo Rico used lacquer but I'm unsure if they used some kind of stain before applying it. When I tried using teak oil before it was too dark.
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Old 12-09-2015, 19:02   #6
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Re: Lightweight but strong backing material

AFAIK Cabo Rico just varnished the teak.
They only used plantation teak which is lighter than the real one, so it should not be difficult to sand and coat it to match the rest of the cabin. Go with 120, 180, 220 grit and dose the varnish accordingly.
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Old 12-09-2015, 19:08   #7
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Re: Lightweight but strong backing material

It isn't the same as marine ply. In addition to waterproof glue marine ply has to be void free. Exterior grade allows some voids.
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Old 12-09-2015, 19:21   #8
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Re: Lightweight but strong backing material

Quote:
Originally Posted by SVTatia View Post
AFAIK Cabo Rico just varnished the teak.
They only used plantation teak which is lighter than the real one, so it should not be difficult to sand and coat it to match the rest of the cabin. Go with 120, 180, 220 grit and dose the varnish accordingly.
No, they used lacquer inside... but made their own.

Basically took turpentine and melted styrofoam cups in then added reducer and applied with a rag in a french polishing style. I'm not going to do that... I'm going to spray it.
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Old 12-09-2015, 19:32   #9
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Re: Lightweight but strong backing material

Take a look at Coosa board: Specialized Structural Panels - Coosa Composites

It's structural strength but lighter than plywood, and it can be painted. You can order it from Hamilton Marine.
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Old 12-09-2015, 20:00   #10
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Re: Lightweight but strong backing material

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Take a look at Coosa board: Specialized Structural Panels - Coosa Composites

It's structural strength but lighter than plywood, and it can be painted. You can order it from Hamilton Marine.
I've worked with coosa before but it won't hold a screw real well, or at least as much as we need it to. Our boats used 5200 and screws to keep the port in place and I would like to switch to using screws (or maybe bolts) with butyl until we can afford to upgrade. Others CR's have done this successfully. Apparently we are the last CR on the planet not to already rebed the ports!

We would like to upgrade to New Found Metal ports, which compress around the fiberglass but right now that is not in the budget.
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Old 12-09-2015, 21:15   #11
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Re: Lightweight but strong backing material

Buy real marine ply---Meranti. Coat twice with epoxy thinned 15% with xylol the first time and then full strength low viscosity epoxy. G10 is too heavy and slippery to bond well. Ply will hold screws all the foams are inferior for sinking screws. Then you can cover with anything as a cosmetic treatment. More ply, veneers, plastics, ceramics, whatever.

JMHO
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Old 12-09-2015, 23:07   #12
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Re: Lightweight but strong backing material

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Buy real marine ply---Meranti. Coat twice with epoxy thinned 15% with xylol the first time and then full strength low viscosity epoxy. G10 is too heavy and slippery to bond well. Ply will hold screws all the foams are inferior for sinking screws. Then you can cover with anything as a cosmetic treatment. More ply, veneers, plastics, ceramics, whatever.

JMHO
No, just no....

1) yes use marine ply not exterior grade
2) never thin epoxy with anything. It ruins the bonding strength, and the waterproofing. It makes the epoxy pourus. If coating wood use two coats on neat unthickened epoxy.
3) G10 is epoxy it bonds perfects well if you use epoxy as a glue. It is the backing plate of choice of pretty much every marine professional. It can also be tapped to bolt directly too. If the G10 is thick enough to get enough threads in it.
4) Screws have no place on a boat. It shatters glass, and opens routes for water to ingress. If there is wood under the glass, then it will hold, right up until the water rots out the core. And as mentioned foam won't hold screws at all.
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Old 12-09-2015, 23:26   #13
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Re: Lightweight but strong backing material

Cabin tops are usually curved. Ply bends. Thinning epoxy allows it to penetrate the wood. G10 is glass. If you want more glass why not lay up more glass.

Builder's all do things a little differently and either way would work well.
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Old 13-09-2015, 02:05   #14
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Re: Lightweight but strong backing material

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Thinning epoxy allows it to penetrate the wood.
BS with capital letters. There's no such "thinner" for epoxy..
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Old 13-09-2015, 08:50   #15
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Re: Lightweight but strong backing material

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Originally Posted by Lojanica View Post
Cabin tops are usually curved. Ply bends. Thinning epoxy allows it to penetrate the wood. G10 is glass. If you want more glass why not lay up more glass.

Builder's all do things a little differently and either way would work well.
Thinning epoxy also makes it pourus. I.E. It is no longer waterproof. There isn't a single epoxy formulator anywhere who recommends thinning epoxy for any reason. It destroys the physical properties of the result.

To resolve the curve of the deck issue you use thickened epoxy as a gap filler. The same as you would use to bond plywood. Over short distances you have to do the same thing with plywood, it isn't that flexible.
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