Morris Minor Owners Club

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

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.

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.

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.

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.

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

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....

stuffedpike20 wrote: ↑Fri Dec 12, 2025 8:27 pm 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'.

Stirling board might be an option.

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