There are several products that can work. Marine varnish, actually almost any marine wood coating, Rustoleum oil based paint.
A secondary suggestion for you, if you are going to have a sump inside the stand, build a tall tray from sheet pvc as a way to catch spills, splashes and overflows. You can see how I built one for my 40g cube's sump in my build thread. There is a link to it in my signature under this post. I've done this to my last 4 builds and it's proven very useful. I even go so far as to install a bulkhead about halfway up the back and plumb it out to the back wall of the house and outside. If you have any drains around, you could plumb it to one of those as well.