I have a problem. I'm sealing MDF with PVA before painting it as I'm using water-based paints and heard that's the way to do it.

I was doing fine, painted about 5 boards, when my dad decided to paint one in the shed (it was the last board and he was complaining it was in the way). I can't remember if it was damp outside or not as I was not in the house when this happened. I've returned to the wood on a sunny day, there are now little black spots like moisture has got trapped between the wood and the PVA. Is this mold or something to do with the fibers? Is it safe to keep and paint over or will it keep growing. If it is mold should I scrap the board and start over?