I'm using 3/4" plywood for a diorama base, 24" x 60" in size. How should I prep it before starting the groundwork, would a coat of clear polyurethane be suitable? If so would I do both sides to keep it from warping?

Wow, that is going to be a heavy dio. Do you plan on laying down a lot of Celluclay or something with a bit of moisture in it? If so use two coats of poly on the wood.
Sand the bare wood down some just to make sure it's fairly smooth. Then put on a good even coat of poly. Let it dry throughly. I usually let it sit for 24 hrs. Then sand it lightly again with a finer grain of sand paper and put down a second light coat. That should seal it just fine. And only the "working" side needs to be finished.

Keep us posted.
Cheers. BillK.

Hi Barry,
I use wall filler as the base of my dio's and found out the hard way that unless you score some lines into the wood, the plaster likes nothing more than to slide off the base if knocked later on. I score a criss-cross pattern on my bases before adding the groundwork and find it holds it fine.
Not sure if the same applies for other mediums, but just in case.

Cheers,

James

Plywood doesn't suffer from warping as much as a piece of hardwood. Seal the top to keep the groundwork moisture from the wood.

An associate of mine made a road diorama about the same size as yours for his 1/25 trucks. He used either 3/4" or 1" plywood, didn't seal both sides, and over the winter the thing twisted like a pretzel. Because your doi will be long & skinny, I'd seal everything.

Jim

Where is the diorama going to be stored? I've got two that have gone 7 - 10 years and not a warp in sight. Mine have been inside and in a non-climate controlled garage.

The diorama will sit on two base cabinets (2' x 3' side-by-side) which I bought for the purpose of displaying it. As it's so large it won't be transported to contests, but rather will sit in my new hobby room that I'm currently building. I'll either secure it from below with a couple screws or tack it in place with a few SMALL dabs of PL Premium adhesive.