Glassed plywood vs. plain plywood

[Guy]

Dear All,

What are the strength comparisons between fiberglassed plywood and plain plywood? By way of example, can a 3/8" piece of glassed ply substitue for a 1/2" piece of plywood?







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[Guest]

Guy,
Are you talking about just glassing the exterior side?
(I'm going to summon the "expert", he's usually pretty fast at responding)

[SteveH]

I'm no expert but I have done some fiberglass work, and I would say it depends on how many layers of glass cloth and the weight of the cloth you apply. It would be very easy to make 3/8 ply stronger than 1/2" with enough layers.

[Guy]

epoxy on one side glass and two coats on the other.

SteveH how much stronger would the 3/8 glassed be than plain 3/8?

[Arne]

Given that, I think the 3/8 will do...

Even brushing resin on adds a lot of rigidity to ply.....

Slow curing resin will soak into wood and make it much more durable....

[SteveH]

"How much stronger?" Hmmmm, that's sort of like " how long is a rope?"

Again, it would bepend on how many layers of cloth and the weight of the cloth layers you apply. It wouldn't be difficult to make it twice as strong. If I were going for strength, and weight were not an issue, I'd put two layers of that heavy mat type fabric, followed by a layer of light cloth to help cover the weave. That's the way they make boat hulls strong enough, by using layers. The cheap fast way is to use a chopper gun, but that is not practical for home application.

[Guest]

I evolve in the canoe and kayak building world so the info I have is related to thinner wood than what you would probably use on a trailer. Glassed plywood will be more puncture resistant than non-glassed plywood and not just more abrasion resistant. As an example, 1/4 ply with one layer of 4oz glass plus epoxy on both sides will withstand a pressure of 429 pounds before failure. The same ply without glass and epoxy will fail with 225 pounds of pressure.

I hope this helps a little.

PS: This link shows results of some tests done on thinner plywood but it can give you an idea of the extra strength added with glass/epoxy: http://www.thag-o-mizer.net/Plywood.html

[Dave M]

The real question is, to what purpose? For a TD glassed outter ply seems a bit unnecessary. If your omitting the aluminum skin than just epoxy coating will do to seal the ply. Your not likely to see many punctures to your TD and if you do, ply is easy enough to repair. Id worry about the weight gain needed to get the 3/8 to match 1/2. It may even exceed the weight of the 1/2" ply. Not to mention the work and expense involved in laying cloth and epoxy.

[Guest]

I see your point Dave, but what I think is key... Do you just seal the exterior side of the exposed woody panels, or do you seal the entire woody skin on both sides?
Since I'm going to be glueing up 1/4" x 3/4" bead and cove strips onto a 1/4" plywood profile base, which will be attached to my stud wall framing with 1" insulation, I was thinking that I could just get away with epoxy on the outer surface of my bead and cove skin.
Glen isn't too sure about that though, he says that over time... humidity, condensation that creaps in from the interior can warp my bead and cove strips and that the outer layer of epoxy wouldn't be strong enough to keep the cedar strips from warping.
Sealing the entire wall on both sides would be an easy step to undertake if
if my walls weren't going to be insulated.
One thing that I didn't mention about my composite wall... I'm going to glue and screw the 1/4" plwood profile base to my stud wall, prior to glueing on the bead and cove strips. I don't want any exposed screw heads or filled countersunk holes for that matter showing on the bead and cove strips.
I would like to know if I can rely on the glue that holds the bead and cove to my profile base to act as my interior side sealer.

[Dave M]

Epoxy sealing both sides of the outter skin element cant hurt. You could probably get away with epoxy sealing the outside and just using good old house paint on the inside.

Are the cedar strips going to be the outter surface of the TD? Like a cedar strip canoe with backing? If so... well Id want to coat them real good, maybe use epoxy to glue them down to the backing. Cedar like western red cedar is pretty dimensionaly stable but it does like to absorb water. Id probably epoxy coat my base layer on the outside only, 1 coat, and house paint the inside to seal it (saves some money). Glue the strips in with epoxy, then give the whole thing 2 coats epoxy to seal the wood on the outside and finsish with some varnish for UV protection. Re-apply varnish every couple years.

[doug hodder]

Guy...I've followed your questions and I would have to agree with a lot of the comments here. Previously I had mentioned cloth mostly as an abrasive protectant, and I completely blew off the puncture resistance qualities of the glass. That didn't even cross my mind as I'm such a panty waist with my boats, sandy beaches are my issue. You need to ask yourself if you want to drop the dough and time on doing the glass thing or just go ahead and get the heavier plywood. I would think that if you are wanting to build tears for the vets, that the money could be better spent. I can't imagine that there is going to be any sort of weight advantage that would cover the cost and time. Also, I think that you could get away with just resin or cloth and resin if you choose, just on the outside of the tear. I do encapsulate my boats, but then you will end up with water on the interior of the hull, If you don't do this on a boat, or seal off the plywood on the interior of the hull, water will soak into the ply, get trapped between the wood and the exterior coating causing a cloudy appearance. I just don't see this as a problem on a tear. Doug Hodder

[Guy]

Thank you all for your posts.

I even called Larry at Raka. He was extremely helpful. Other than the abrasion or punture resistance he suggestednot even glassing or at the very most he said to use 3.25 oz. glass. Larry said that the glass would not add strength to the plyood and it would add more weight than increasing the plywood. He did point out the abrasion resistance but said the clear coat over a seal epoxy coat would be just a little less and the plywood is as easy to repair as the fiberglass.

[tjdale]

Why not just epoxy coat the roof and sides and use glass tape on the junction/seam between the two? This is done on a number of tack-and-tape hard chined plywood boats. This would make a strong, waterproof seam without using any sort of edge banding.

[jimqpublic]

I read an article a while back on the whole issue of why does epoxy work great for boats but not always for RV repair. The conclusion is that with boats the wood is generally encapsulated, the humidity level constant, and the atmospheric pressure pretty constant. With RV's the epoxy is often used as a glue or to coat just one side. Humidity level is highly variable, and pulling up over mountains causes huge changes in atmospheric pressure. The wood is swelling and shrinking which works against the very strong, rigid epoxy causing failure.

So I say coat both sides and use at least a light layer of glass on the outside if you're going to paint or varnish. You won't even see the glass when you varnish (I built a Kayak this way). This will prevent crazing or cracking of your final coat. "House Paint" will slow down the swelling/shrinking due to humidity but not stop it entirely.

Regarding strength- Personally I think 3/8 ply is plenty strong for a TD. It may not be as stiff as you want, even after adding epoxy. If I ever do a TD it would be 1/8 ply sandwiching 1" or more of foam core. This structure will be stiffer than even 3/4" plywood, weigh much less, be "strong enough", and insulate very well. More trouble to build though.

[tjdale]

I have to disagree somewhat. I think a plywood with multiple plys would be fairly stable if just encapsulated on the outside. If this is a worry why not just tape the outside seams and seal the surfaces of the wood with paint. This is done on alot of tack and tape boats. Payson does all his boats this way and I don't think he has ever reported a problem due to humidity/wood movement. And if you are using a framed wall/ceiling that insulated and skinned both sides I think the difference in humidity levels would not be that great.