I concur on the polyurethane sealant. It will take a lot, as MDF is thirsty stuff.
I built a subwoofer a few years back with two 3/4" MDF disks that I glued & pressed together. Routed the edges, painted it black and then sealed the hell out of it. It still thumps with the best of them and repels liquid spills with no problem. It's a decent amount of work, but it's doable.
If there is one thing a remote-controlled, silent and unseeable surveillance/killing machine needs, it’s more whimsy. -- Marcus