Plywood moisture sealer?

If it's the inside of the stand my prefference is white primer with either white deck paint or polyacrylic. The more coats the better.
 
I've been lurking on here as I prepare to build my 75 gallon aquarium. In my woodworking and freshwater experience, Spar urethane has shown good waterproofing properties. It comes in paint/roll on or spray on applications, and is as easy to work with as the typical polyurethane.
 
For inside the stand I use a couple coats of primer, then a few coats of white latex exterior. I also seal the bottom several inches with flexseal.

For the outside of the stand it depends on your plan. Primer and exterior latex if you decide on paint. I prefer the look of wood, however, so I use stain and then several layers of General Finishes exterior polyurethane. It's water based, so almost no smell as it cures. I used it on a neighbors Green Egg cart about 10 years ago. It's held up remarkably well, although he does cover it between uses.
 
I'm going to replace my old top deck with the new one I made of playwood. I'm wondering if I need special preparations before sealing it? I'm planning to use Waterlox Original Sealer (https://woodyexpert.com/best-deck-sealer/), it is stated that the product is self-sufficient and will be waterproof. I'm also wondering if I need to seal both sides?
 
I'm not familiar with that, but yes, seal every surface you can, and with at least a couple coats. More is better. The last thing you want is for the plywood to get even damp and start delaminating. Sealed well, it will be impervious to pretty much anything.
 
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