To paint over ANY surface, without sanding, use a product called 'ESP Easy Surface Prep', available at hardware and paint shops. Wipe on, wipe off and paint in 90 minutes. Works well over enamel paint , without having to sand. SAVES HOURS of work. It was recommended to me by a professional painter.
We purchased 4x8 foot sheets of plywood bead board. I do not recommend the large compressed fiber board sheets. Granted they are lighter to work with, but it chips and dents easily, and doesn’t look as authentic. We also purchased all the trim for crown molding. We primed and painted all the wood work before we made a single cut. Painting overhead is no fun. FIND YOUR STUDS!