Did you try putting Gorilla Tape on the back side of the panels? That's what I do in situations like this, and it keeps the light from coming though from the back. You can even double-layer it if you need to -- and you don;t have to worry about it not accepting paint.
In fact, we did after we put the Gorilla Tape on the front. We put one or two layers on the back, as needed to block the light from shining through.