alternative material.

[stuffedpike20]

I had a structure to build outside (won't bore you with the details).
I went to B and Q and bought some 12mm exterior ply wood. I unibonded the edges and gave it 4 coats of varnish.
It has lasted about 6 months. It's de-laminating and spongey and growing fungi.

I thought exterior ply would be ok outside, but it is not.
Is there an alternative sheet material that would last better, but not cost too much? Is there a better ply-wood?

Thanks
John

[Paladin1962]

I've found that the chemicals used to "weatherproof" plywood sometimes don't dry out completely, then evaporates as it ages, lifting any coating and/or rotting the wood away. I try to find the driest piece available when I buy it; but this isn't easy if you have to buy several or have them delivered.
A life expectancy of only a few months raises alarms... that stuff isn't cheap on a good day.

[Bill_qaz]

I believe there's an actual marine ply used in boat building.
Maybe talk to an actual timber merchant for advice.
Unfortunately B&Q and there ilk are probably more focused on low prices.

[unclealec]

I built a workshop during lockdown ( I call it the lockdown lock-up) using a steel frame and reclaimed timber studding clad with secondhand box profile roof sheets.
Then I lined it with secondhand Kingspan reclaimed from buliding site offcuts, and I have to say that it is astonishingly warm, dry, cheap, and comfy.
My point being - if you clad the ply with cheap box profile it will keep the weather off and probably ward off the problems you speak of.
Mind you, I would have gone for 19mm shutter ply.

[stuffedpike20]

Thanks folks.
Yes, maybe a proper timber merchant would be a good idea.
I only need about half a sheet, which is why I was thinking of a plastic or something...but top grade ply is probably the best option.

[michael4]

OSB or Sterling board and then paint it using any old paint that you can find in your garage.

[geoberni]

There's a lot of different grades of 'exterior plywood', even somewhere like B&Q.

See this guide for example: https://elliotts.uk/ideas-and-advice/ch ... od-a-guide

But I think your choice of edge treatment is a significant factor in your problem....

I unibonded the edges and gave it 4 coats of varnish.

That covers a multitude of substances, since Unibond is a product range of 50 or so substances, from sticky tape to gap sealant.

There are specific Panel Edge Sealant products for such panels, I don't thing Unibond have anything suitable.
You need to search for 'Panel Edge Seal Protection'.

[stuffedpike20]

Stirling board might be an option.

You are right berni, PVA on the edges was not the best idea.