Sealing wooden bases

Warping is aonly a major iussue when you use a single plank of wood. If you use plywood its not really an issue as each layer has the grain running at 90 degrees to the layer above and below.

If you are going to coat the wood with a varnish type material, do it all over, top bottom and sides. The idea is to limit the penetration of humidity into the wood.

You can use a good water sealer, like Thompson's Outdoor Water Sealer, and just for good measure and extra insurance, hit it with a heavy coat of acrylic sealer after it has thouroughly dried.

Or, use shellack, and then seal that with an acrylic sealer. I've been pretty successful with both of these methods!