Plywood glue weakening

[edgeau]

I am lucky enough to have a specialist plywood merchant in my area. Same prices as the big hardware warehouse chain but you get to talk with someone knowledge. I was in there to pick up a sheet for another project and was chatting about structural ply. The glue is outdoor rated but not boil rated like marine ply. No surprise there, but when talking paint / varnish he told me water based only as oil based ones degrade the glue!
That would be a bummer if you got this stuff and used the mix of spar varnish that is so common among us. I have used it extensively but my TD is marine plywood so a different glue.

I have a little left over so maybe an experiment is in order... Might take months to get a result though.

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[Socal Tom]

Its been a concern of mine that the Poly/Mineral spirits mix might be bad for the glue . Looking forward to your results.

[MickinOz]

Ouch. I sincerely hope this isn't so, but I have seen it mentioned before.

My first build is structural ply with A-bond glue and has been heavily soaked in the Mix.
The plywood came from a mix of suppliers.
Some is Arauco Plywood from South America that I got from the Mitre 10 store in Port Augusta.
It was stamped "A-Bond" and they say on their website that all Arauco plywood is manufactured from plantation grown Radiata Pine glued with "WBP A-bond Exterior Phenol-Formaldehyde resin."
That is the side walls, floor, kitchen bench and interior cupboard floor. It all got soaked in the Mix.

The roof is Ecoply structural ply from Bunnings at Parafield. This is stamped as having an A-Bond.
I discussed structural ply with a representative of Carter Holt Harvey who supply Bunnings with the Ecoply.
He said the glue is the same as marine ply. In any case, that got soaked in mix too.

I put Zinsser Cover Stain primer over this (oil based) and then Solver Exterior Oil-based enamel.
I have noticed no delamination after 10 months of quite regular overnight trips. Winter being a great time to camp here, its been wet a few times from rainfall, too.
I note that the face veneers of several raw off-cuts that have been out in the weather for about 18 months have degraded badly. Checked, split, peeling. That there mix is very important, I think. It seems to me that the one of the very significant things in Australia is that unless you try much harder than the Big Green Shed, you are going to get plywood made with Radiata Pine. When you look at electron microscope pictures of the end grain, the pores are huge and more numerous than the pores in hardwoods. I reckon the sponge like nature of the wood makes the Mix imperative.

My second build is using 12mm ply for the floor and side walls that is stamped as BS1088 marine plywood.
I gave up on trying to bend the rather poor quality 6mm I bought for the roof. Some of that wasn't WBP when I boiled it I found, and it was just plain too stiff to bend with its very uneven veneer thicknesses.
So off the 'guta again. The "7mm structural" plywood I bought up there had "A-bond" written on the price tag on the racks, and "Arauco" and "6.5 BC WBP" stamped on it.

When I get to cutting the roof, I will be able to boil some samples.
I'll go with 12mm marine ply, the WPB stamped 6.5mm Arauco ply, and some of the materials I used for the first camper.
I'll do a piece of each "raw" and sealed with the Mix.

Or maybe I might get netter peace of mind not knowing.

[tony.latham]

Construction grade plywoods in the U.S. use urea-formaldehyde glue. I've been varnishing the stuff for at least fifty years without any degradation. In recent years, I've used thinned polyurethane with the same results.

Any idea what glue is used in the stuff in Australia?

Over here, there's a cheap plywood called luan that is sold in the box stores. I know of one builder that used CPES epoxy on his luan and it caused it to delaminate. That stuff uses alcohol, xylene, and toluene as thinners.



Tony

[MickinOz]

Tony, the following comes from a Australian timber supplier's website:

"Plywood Manufacture
Four types of glue bonds are used for plywood manufacture, in decreasing order of durability.

Type A bond uses a phenol-formaldehyde resin which is specified for marine and exterior plywood and will not weaken under wet conditions, heat, or cold."

"Structural Plywood

Structural Plywood is suitable for structural applications as a Type A bond is used."

This below comes from the Engineered Wood Products Association of Australia:

Plywood.JPG (56.27 KiB) Viewed 501 times

I look though. I want to see that dark glue line, and if it's the real deal, it has the stamp on the back saying what the glue is, and what standard it conforms to,
I believe there has been imported substandard plywoods that failed.

[MickinOz]

So, in theory at least, the same glue is used in both marine and structural plywood. So they say.
I always read the fine print on the back of the sheet.
However, I have definitely seen prior mention of this issue that edge has raised, on this forum I think.
I reckon it was about the glues being changed to less toxic materials and this resulting in lesser solvent resistance.
I always check - I have to see that dark brown, almost black, glue line and I have to see A-Bond or WBP printed on it.

BTW, I did put in one piece of 3mm "appearance grade interior" plywood in #1. It formed the internal skin on the front curve, and had bright pink glue line.
It got saturated with the mix, too. Didn't fall apart yet, but that may not mean much. It could have weakened it but its not under much stress and it may never actually fail.

[tony.latham]

So, in theory at least...

Sounds good to me.



Tony

[Tom&Shelly]

I have a little left over so maybe an experiment is in order... Might take months to get a result though.

I used a lot of (American) AC home construction grade plywood in our galley cabinets, and varnished them all with oil based varnish for water-proofing. Did it about a year ago, and they seem to be fine.

This past summer, I found a couple of tool boxes my dad built out of scrap ply from their house build in 1960. They were varnished (oil based, I'm certain) and are holding up well. He used them as "field boxes" to hold fuel, starter and tools for his RC model airplanes. They've seen a little rain now and again, a lot of sun, and more than a little caster oil used as both fuel and cleaner. No evidence at all of the varnish attacking the glue.

Tom

[MickinOz]

I used a lot of (American) AC home construction grade plywood in our galley cabinets, and varnished them all with oil based varnish for water-proofing. Did it about a year ago, and they seem to be fine.

This past summer, I found a couple of tool boxes my dad built out of scrap ply from their house build in 1960. They were varnished (oil based, I'm certain) and are holding up well. He used them as "field boxes" to hold fuel, starter and tools for his RC model airplanes. They've seen a little rain now and again, a lot of sun, and more than a little caster oil used as both fuel and cleaner. No evidence at all of the varnish attacking the glue.

Tom

Trouble is, and I think this is where Edge's informant would have been coming from, "past performance is not a guarantee of future returns", as my investment advisers are fond of pointing out.
That's why I always look for the correct label. If it's cheap imported stuff with no stamp, you could be buying anything.
Vigilance is key.
I'm jealous that you can get AC construction grade plywood.
We don't see any of that.

The EWPAA document I got the bit about the A-Bond from also recommends acrylic latex paints for plywood. The reason given is that oil based enamels don't flex enough when the timber moves due to moisture. I console myself that, if the mix has been generously applied, then my oil based enamel paint won't need to flex.

Page 248 if you are interested:
https://ewp.asn.au/wp-content/uploads/2 ... INAL-1.pdf

[edgeau]

]

I look though. I want to see that dark glue line .

This stuff does have that nice dark line.

I'll make a few marker stakes for the veggie garden - being stuck in wet soil has to be the worst condition for it. Then we will see if there is a difference between treatments.

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