I'm the Straw Man in your argument
Nah, I don't think that's the case.The most recent piece I bought at Home Depot (which was 4' x 8' and yes I did bring my hobby knife with me to cut it up into pieces that fit in my car in the parking lot) had a thin film on it that I need to peel off. It cut easily, so I didn't realized it until after I had started shaping it, but thankfully before I applied mod podge or paint.
I just got home from work and checked it out. I think the problem is that it's slightly raised in the center and I was using quite a bit of water. I suspect the paint was moving away from the center and flowing to the sides because of that.
I'll put another coat on that one, but use a book or something to flatten it out. Paint one half tonight and the other tomorrow.