Minimum Woven Glass for Moisture Proofing

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TomTom
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Minimum Woven Glass for Moisture Proofing

Post by TomTom »

What is the minimum weight of woven glass that is considered "totally impermeable" to moisture/ water over plywood?

I read some where it was 3-4 clear coats of epoxy.

If thats the case, I feel like using thin woven glass everywhere would be easier. The question is how thin can the glass be!



cracked_ribs
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Re: Minimum Woven Glass for Moisture Proofing

Post by cracked_ribs »

I don't know the answer, but personally, I'd be comfortable with 2oz on interior surfaces and 4 on exterior.

But that is based on absolutely nothing.

Still if it's getting 2oz fabric, then paint...I don't know, that's pretty impermeable. Plenty of boats out there with nothing but paint, and they often hold up fine as long as the paint keeps getting refreshed.

But if it were a surface that was underwater on a boat on a mooring year round...yeah, no idea.

On a trailer boat, interior surface, I'd be fine with 2 and will likely use 4 only if the local supplier doesn't have 2.
I designed my own boat. This is the build thread:

viewtopic.php?f=12&t=65349

fallguy1000
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Re: Minimum Woven Glass for Moisture Proofing

Post by fallguy1000 »

TomTom wrote: Wed Feb 17, 2021 11:16 am What is the minimum weight of woven glass that is considered "totally impermeable" to moisture/ water over plywood?

I read some where it was 3-4 clear coats of epoxy.

If thats the case, I feel like using thin woven glass everywhere would be easier. The question is how thin can the glass be!
Well, you are not going to like to heard this, but Richard Woods told me 1208 tapes are the minimum to be considered impervious on a floor. This is due to wear.

Now, I would think 3 layers of 6 oz would be close to equal. I have a canoe with two layers of 6 oz on the bottom and inside and one layer on the sides and two near coats and we dinged it real good and broke the skin once. But based on that, a layer of 6 oz and two neat coats is pretty solid, unless it is getting walked on; then I'd say add a layer of glass.
My boat build is here -------->

viewtopic.php?f=12&t=62495

TomTom
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Re: Minimum Woven Glass for Moisture Proofing

Post by TomTom »

Fallguy - are you sure that that is for plywood and not foam? Seems an awful lot to me and well above anything spec’d in plywood composite plans sold on this site.
cracked_ribs wrote: Wed Feb 17, 2021 11:46 am I don't know the answer, but personally, I'd be comfortable with 2oz on interior surfaces and 4 on exterior.

But that is based on absolutely nothing.

Still if it's getting 2oz fabric, then paint...I don't know, that's pretty impermeable. Plenty of boats out there with nothing but paint, and they often hold up fine as long as the paint keeps getting refreshed.

But if it were a surface that was underwater on a boat on a mooring year round...yeah, no idea.

On a trailer boat, interior surface, I'd be fine with 2 and will likely use 4 only if the local supplier doesn't have 2.
I am starting to feel that it’s almost as easy and much better protection to do this rather than just clear coat. And I don’t think resin usage would go up too much?

fallguy1000
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Re: Minimum Woven Glass for Moisture Proofing

Post by fallguy1000 »

TomTom wrote: Wed Feb 17, 2021 1:15 pm Fallguy - are you sure that that is for plywood and not foam? Seems an awful lot to me and well above anything spec’d in plywood composite plans sold on this site.
cracked_ribs wrote: Wed Feb 17, 2021 11:46 am I don't know the answer, but personally, I'd be comfortable with 2oz on interior surfaces and 4 on exterior.

But that is based on absolutely nothing.

Still if it's getting 2oz fabric, then paint...I don't know, that's pretty impermeable. Plenty of boats out there with nothing but paint, and they often hold up fine as long as the paint keeps getting refreshed.

But if it were a surface that was underwater on a boat on a mooring year round...yeah, no idea.

On a trailer boat, interior surface, I'd be fine with 2 and will likely use 4 only if the local supplier doesn't have 2.
I am starting to feel that it’s almost as easy and much better protection to do this rather than just clear coat. And I don’t think resin usage would go up too much?
Richard said 1208 min on a walked on surface.

But my canoe has 2x6oz woven plus 2 neat coats on the floor and never wore thru. I varnished it as well.

Like I said, for non-contact, the sides of the canoe are 6oz woven with two neat coats and have held up 20 years with varnish.
My boat build is here -------->

viewtopic.php?f=12&t=62495

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Re: Minimum Woven Glass for Moisture Proofing

Post by jacquesmm »

Fiberglass does not seal anything: water and moisture go through glass. It is the resin that seals the panel. It will never be 100% sealed but for all practical purposes, even the thinnest layer of glass and resin will seal sufficiently tp keep the water out. We can debate how thick that minimum should be but in all cases, sufficient glass to resist some abrasion or provide stiffness will be more than enough.
That is for my designs, the situation may be different for wooden boats.
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http://boatbuildercentral.com

cracked_ribs
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Re: Minimum Woven Glass for Moisture Proofing

Post by cracked_ribs »

TomTom wrote: Wed Feb 17, 2021 1:15 pm I am starting to feel that it’s almost as easy and much better protection to do this rather than just clear coat. And I don’t think resin usage would go up too much?
That's my preference, for sure...I don't think resin use changes much at 2oz; it might not even at 4oz. It might even be lower because I think it's easier to get an even, thorough coat when wetting fabric.

As Jacques says, it's not really the fabric; it's the epoxy that seals it. The fabric is just there to provide abrasion resistance and to hold the epoxy in place as it kicks. An epoxy coating that's one mil thick is essentially waterproof, it's just that minor damage would break the membrane. Three coats of epoxy is just more resistant to damage. But a bit of cloth is even more so.

You lay down light fabric, it's very easy to keep it all sealed up. It's a tiny bit heavier but personally I prefer it and am willing to eat the extra 6 pounds or whatever spread across the entire boat.

I put 4oz fabric on the sole of a little sailboat I built around 20 years ago. It held up perfectly. If I wore cork boots on deck, 3x 1708 wouldn't be enough. It just depends on what you're doing. But as far as waterproofing goes, if it's on the inside of the hull or on a bulkhead or whatever, I'm inclined to put a really light layer of fabric on it just to get the coat thorough and even, and make it resist an errant sinker or something a little better.
I designed my own boat. This is the build thread:

viewtopic.php?f=12&t=65349

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