I have a junky piece of furniture made of laminate that could be made into a really nice enclosure, but I want it to be humid so I was wondering if I could just seal it with polyurethane....................but it needs to be water tight enough to not rot with standing water in it. Would polyurethane work?

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"Procrastinate today don't put it off!"

Polyurethane is good for sealing wood against regular humid conditions but I would not expect it to take being consantly underwater.
What do you mean by "standing water"? As in a pool? If so you would need some sort of container or to line the bottom and the sides intended to be underwater with glass or plexi.

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Merlin,
What's Life Without A Little Magic!

The sealer will flake off the surface of the melamine. Usually melamine furniture is on of the lowest grades of melamine. A very thin surface of paper on top of the particle core. I wouldn't recommend it for a humid enclosure. I recently experimented with it. I made a melamine box and siliconed all the corners then left some water in the bottom for a week. The melamine surface started to bubble. This wasn't the cheap kind of melamine either.

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Mike
"Never worry about numbers. Help one person at a time, and always start with the person nearest you." - Mother Teresa

could I line it with sheets of black plastic? i mean like those tarps they cover gardens with? as far as standing water I mean like a drainage layer in the substrate like clay pellets where the water will seep down into

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"Procrastinate today don't put it off!"

Nope you are going to need an actual water basin like a vinyl tub. Any wooden surface that is constatly exposed to standing water will rot.

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Merlin,
What's Life Without A Little Magic!

You could fiberglass cover it, it would be really expensive, and would probably be cheaper to just get a nicer material enclosure. I built my Iguana enclosure out of an old dresser, and coated the inside with floor tiling, but it was an oak dresser. Good luck

What I've done in the past is to cut glass to fit the bottom and up the sides. Then silicone all the cracks. It's just like a build in aquarium. Worked great for me last time. Make sure to silicone the top edge as well because it's very sharp. Glass is fairly inexpensive if you cut it yourself.

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Mike
"Never worry about numbers. Help one person at a time, and always start with the person nearest you." - Mother Teresa