" but it only takes a deep scratch to turn it into a sponge. "
So it "can" become a painted sponge. In the end, it's just not the material of choice in humid or wet applications. It's biggest claims to fame is relative stability and ease of finishing; meaning it won't shrink and swell wildly the way open-cell woods like pine do and it's a snap to paint. But it's just a matter of time before it develops some break in whatever coating is applied to it and you're done.
I just bought a set of the metal shelving units that Lowe's sells touting 1,000 lb per shelf capacity. I replaced the MDF shelves with 3/4" plywood. The MDF shelves have a really interesting coating. Not paint, yet not epoxy. I stood them on end in the driveway until I could haul them away. In the ensuing monsoons that came that week, I have to say I'm impressed with whatever they coated that MDF with. It appears to be pretty tough and waterproof.........except at the uncoated edges. They didn't even sit in a puddle and were barely making ground contact on the gravel. The bottoms of the panels swelled to 4 times the panels' thickness about 2 inches up. But the rest of it was relatively unaffected by about 6 inches of rain.
"Why can't my wife see all of this stuff as an investment?"