Sealing birch for underwater use?

So I found an amazingly beautiful chunk of wood outside earlier this year that's almost exactly the right size and shape to use as a background for my 125 gallon aquarium, but I don't think the wood's going to hold up underwater without some protection.

It may or may not be birch, it has a thin, papery, silvery bark, but I don't know what it's leaves look like because the top was chopped off before I discovered it. It was growing in a swampy area, so the roots have a lot of character to them, the whole thing must have fallen over years ago and regrown sideways, it's hard to describe, I'll try to take a picture tomorrow.

What I want to know is, is there a way to preserve this thing properly, so that it can remain submerged for a long period of time without falling apart? I'm also hoping to be able to grow some plants on a couple sections of it, but if I can't, I'll be fine with growing plants in the substrate/attached to rocks.

What I was thinking about doing, was hollowing out most of the bigger chunks (I'm hoping to hide my equipment behind this thing), and possibly counter weighting it with some cement or something before I seal it and glue it down with silicone. I know I can get it to stick pretty well without counterweighting it, but I'm paranoid of it somehow exerting enough force and one day blasting up out of the tank and probably exploding it. I'm pretty sure that's a stupid and irrational fear, though.

Anyone know a fish safe coating that I could use for this project? I was thinking that if the coating makes the surface too slick for plants to attach, I might just mix a little sand into some of the areas I want plants to cling to. I think I read somewhere that placos and other animals with similar feeding mechanisms are harmed by feeding from the surfaces of treated wood so I'm not planning on keeping any of them.

Any other suggestions?